Medical Immunology and Microbology, Part 1

Barriers to Transplant

2 main barriers: 1) there are not enough available organs 2) immune response/non-self recognition

Properties of IgA & SIgA

2 subclasses of IgA: IgA1 and IgA2 IgA in blood is monomeric, most of IgA in blood is subclass IgA1 SIgA is dimeric, and in secretions, amounts of IgA2 are significantly increased so you have about 50/50 IgA1 and IgA2 SIgA is highly glycosylated because they include a J chain and an SC, both of which are rich in carbohydrates IgA is a non-inflammatory Ig because IgA wants to be able to dispose of Ag without causing inflammation, which can cause collateral damage to barrier epithelium ∙functions include aggregation, neutralization, and opsonization

IgA

2 subclasses: IgA1 + IgA2 primarily found in bodily secretions, typically in dimerized form →found in blood as a monomer is small quantities often body's first isotype exposed to antigens in its initial interface with external environment as the result of a high turnover rate, it has the highest rate of synthesis

Antigen Presenting Cells (APCs)

3 major types of APCs: dendritic cells, macrophages, B lymphocytes basic method by which APCs act is first by phagocytizing pathogen →pathogen broken down to component peptides, peptides loaded onto MHC cells, MHC cells take peptides and display them on surface, so that T cells are able to interact with antigens

What makes tissue foreign?

ABO Blood type MHC Class I and II Minor Histocompatibility Complex - other polymorphic proteins that people express

AIRE (autoimmune regulator) gene

AIRE is required for expression or presentation of tissue specific proteins in thymus epithelial cells and for deletion of T cells that recognize self absence of AIRE results in self-reactive T cells leaving thymus, resulting in autoimmune polyendocrine syndrome

T Cell Signaling & Adhesion Molecules

Affinity of TCr for MHC-peptide is low and TCR cytoplasmic tails are not designed for signaling as such, TCR needs both adhesion and signaling assistance from others molecules such as CD3, CD4 (binds MHC II) and CD8 (binds MHC I)

General Events in Newly Activated T-Cells (Beyond "Priming Signals")

After a T-cell receives priming signals from an APC, it is induced to secrete IL-2 (T-cell growth factor) and to express and upregulate a high-affinity IL-2 receptor (heterotrimer) IL-2 feeds back, binding to high-affinity receptors on T-cell that secreted it, inducing cell to enter cell cycle and differentiate and generate a clonal population of T-cells After T-cells proliferate, they are induced to increase expression of CTLA-4 (anti-costimulatory molecule) ∙it binds to B7.1-B7.2 molecules on surface of APC's with 4-5 times greater affinity that CD28 (co-stimulatory survival signal), regulating T-cell differentation Once you produce an effector T-cell, it no longer needs co-stimulation, all it needs to recognize an infected cell is the antigenic peptide expressed in the context of MHC

Early stages of B & T cell development are similar

Aim is to ensure diversity of antigen-binding receptor repertoire expression of an antigen receptor on the cell surface is a critical developmental checkpoint Test receptor for antigen-recognition against molecules in immediate environment, determines whether the cell lives or dies B-cells are easier to develop, as receptors function through binary interactions (meaning that B cell receptors work through a simple process of recognizing antigen and binding to it) T cell receptors work through ternary interactions, which means it recognizes antigen peptides in the context of MHC receptors

Antigenicity vs. Immunogenicity

All immunogens are antigens, but not all antigens are immunogens →all macromolecules that make up our bodies could technically be considered antigen, so it is important to distinguish between antigens that are immunogenic or not As a molecule's ability to bind specifically to a membrane bound antibody increases, we say its antigenicity increases Just because because a TCR or BCR is engaged by its antigen does not mean that an immune response will occur →therefore, an antigen's immunogenicity is a measure of how able the substance is at provoking an adaptive immune response against it →only immunogenic antigens can bring about production of antibodies against it

Paratope

Antigen binding portion of an antibody

Plasmablast

As B-cell goes from naïve B-cell to plasma cell, it encounters transition state → plasmablast think of plasmablast as transition cell with advantages of both naïve B-cell and plasma cell

Graft Sources

Autograft ∙from self ∙would if occur if you had something like a myeloma, where you could have some of your own bone marrow harvested, frozen and then re-introduced after chemotherapy and/or radiation Syngeneic graft ∙if you have genetically identical twins you can do this Allogeneic graft or "allele different" ∙graft is from genetically different person of same species ∙could be family or unrelated Xenograft ∙from another species ∙generally recognized and destroyed immediately

MHC Clinical Correlates

Autoimmune/drug toxicity/infectious disease ∙different HLA alleles influence peptide-based vaccines ∙people who have DR4 MHC allele are more susceptible to rheumatoid arthritis ∙some HIV drugs can cause life threatening immune response if you have MHC allele B57 Transplantation ∙unwanted immune responses may occur

BCRs vs. TCRs

B cell receptor is membrane bound antibody, each contains two receptors of identical specificity so we say it is bivalent →specificity of each receptor is created by variable region (distal 120 AAs) of light and heavy chains, which are linked by disulfide bonds →variable region of light chain and heavy chain that constitutes each receptor is denoted the antigen binding domain T cell receptor is univalent →TCR is comprised of 1 alpha and 1 beta chain, the variable regions of which create the specificity of the receptor molecule →in contrast to antibody, the TCR may not be secreted and is always bound to the membrane of T cells →don't let structural similarity of BCR and TCR trick you into thinking that TCR doesn't see its epitope in context of MHC, unlike BCR

B Cells vs. T Cells

B cells are developed in bone marrow, migrated to secondary lymphoid tissues, activated by foreign antigens ∙B cell receptors are created (in bone marrow) by random assembly and transcription/translation of gene segments) →although a B cell can have thousands of receptors, on any single B cell, receptors are the exact same ∙B cells display negative selection in barrow marrow →tested in bone marrow on whether or not they bind self-antigens →if they do, they undergo apoptosis →if they do not, they are released into circulation ∙When B cells are activated, they have two major responses: formation of plasma cells, which go on to manufacture antibodies, and formation of memory B cells, which allow the secondary immune response to act more efficiently T cell precursors begin in bone marrow, but do not mature until they reach thymus ∙migrate to secondary lymphoid tissues and are activated by foreign antigens ∙T cell development displays both positive and negative selection →T cell receptors that recognize self-MHC molecules undergo positive selection and receive survival signals →T cells that do not recognize self-MHC molecules are removed by negative selection →only T cells that received survival signals are released into circulation ∙when T cells are activated in peripheral lymphoid tissues, they expand and differentiate into effector T cells →T cells can differentiate into helper T cells, cytotoxic T cells, or regulatory T cells Only about 1-2% of all B and T cells that mature are released into circulation, most are negatively selected and destroyed Lymphocytes make up about 20-40% of total leukocytes that are circulating in blood ∙out of all lymphocytes, B cells make up about 15%, helper T cells make up about 50%, cytotoxic T cells make up 25%, and NK cells make up remaining 10% ∙major reason you see less B cells circulating is because mature B cells spend most of time within secondary lymphoid tissue

B-Cell Space Management

B-cells that bind antigen enter the germinal centers of the secondary lymphoid tissues and begin to proliferate this has the effect of pushing other B-cells that were there previously out of the germinal center displaced B-cells go on to be plasma effector cells or memory cells if B-cell hangs out in cortical region for too long without seeing any antigen binding, it is destroyed and another newly formed B-cell from bone marrow takes its place

Dealing with ABO Differences

Blood transfusion: try to matc hABO and Rh Solid Organs: try and select donors that are ABO matched to reduce risk of rejection

Transplant situations

Blood transfusions Solid organ transplant Hematopoietic Cell Transplant (HPC)

Autoreactive B cells AND T cells are needed to initiate autoimmunity

CD8 alone may be sufficient, but even then, CD4 would be needed to become effector T cell

Tc-Cell (Cytotoxic/Killer/CD8+) Action

CD8+ T-cells recognize and are activated by intracellular (cytosolic) pathogen peptides in the context of MHC Class I on APCs since viruses enter cells and are present in cytosol, this makes Tc-cells the major player in viral immune defense Because Killer T-cells are so good at killing, they require very high levels of co-stimulation ∙generally, APCs cannot provide enough co-stimualtion and require assistance from activated Th-cells bound to its surface ∙accomplished in two ways: 1) Th-cell secrete lots of IL-2, inducing Tc-cell to differentiate and upregulate IL-2 secretions 2) Th-cell induces the APC to increase its co-stimulatory effects on Tc-cell Fully activated Tc-cells kill by releasing cytotoxic effector molecules (perforin, granzymes, granulysin), just like innate NK lymphocytes and by activating Fas pathway

Types & Functions of Effector (aka Activated/Mature) T-Cells

CD8+ T-cells: recognize MHC I holding peptides from intracellular (cytosolic) pathogens ∙Cytotoxic T-cells: kill mainly virally infected cells CD4+ T-cells: recognize MHC II holding peptides from endocytosed/phagocytosed pathogens **HELPER T-CELLS** ∙TH1 cells →help macrophages combat infections →help B-cells produce IgG1 and IgG3 ∙T2H cells →help B-cells make IgE, which helps fight multicellular pathogens, like tape worms ∙TH17 cells →help initiate acute inflammation by recruiting neutrophils ∙TFH cells (Follicular Helper T-cells) →helps B-cells produce IgA and some IgG (class-switching) →can secrete cytokines that either Th1 or TH2 cells secrete **Regulatory T-cells** →suppress T-cell activity, once they complete their functions →especially important in autoimmune attacks

Mechanisms to distinguish self from non-self

Cell-ligand affinity ∙cells with low affinity for self are allowed to pass into periphery ∙others undergo apoptosis before being allowed to enter periphery Ligand concentration ∙high and constant expression of a self-ligand targets the cell for apoptosis Co-stimulatory signal ∙when signal 2 is not presented with signal 1, the cell is indicated as self ∙presence of signal 2 targets the cell for apoptosis

Layers of self tolerance

Central tolerance ∙potential for autoimmunity exists in all individuals due to mechanism in which self antigens are eliminated in primary lymphoid tissues (broad spectrum of selection) ∙if cells escape this mechanism, they are sent to periphery where they can be activated later Antigen segregation ∙refers to immunologically privileged sites such as testis, retina, brain and uterus ∙here, B and T cells either cannot physically pass a barrier, are exposed to IL-10 and TGF-β (immunosuppressants), or tissue fluids do not exit via secondary lymphoid tissue, thereby limiting exposure to dendritic cells and the like Peripheral Anergy ∙signal 1 delivery without signal 2 → anergy Regulatory cells ∙serve to suppress cytokines as not to promote full-on immune response Cytokine deviation ∙cytokines can promote development of Th2 cells that have anti-inflammatory effects Clonal Exhaustion ∙T-cell essentially runs out of team from chronic activation resulting in death of cell Honorable mention: Linked recognition ∙in this instance, Treg cells can inhibit autoreactive T cells that recognize the same tissue or are presented on APC

What's responsible for the homing of lymphocytes to mucosal surface?

Chemokines to chemokine receptors, cell adhesion molecules to cell adhesion molecule receptors, cytokines to cytokine receptors, addressions, integrins, etc

Steps of Phagocytosis

Chemotaxis - phagocytic cells move toward pathogen in response to epithelium and pathogen signals Attachment - phagocytic cells attach using endocytic receptors that bind to pathogens Endocytosis - organism is enveloped in phagocytic cell Vacuole Formation - phagosome (specialized endosome) moves farther into cell where it becomes more acidic, phagsomes fuse with lysosomes containing antimicrobial and degradative enzymes to become phagolysosomes Killing - acidity of phagosome may be sufficient to kill pathogen, phagocyte also consumes oxygen in preparation for respiratory burst, which results in highly reactive O2 and N2 that can further oxidize contents of phagolysosome and killing surviving pathogens Digestion - pathogen is broken down into peptide fragments, after digesting pathogen, cell will take specific peptides from pathogen and display them on MHC Class II molecules on surface membrane of APC

MHC Genetics

Chromosome 6 contains all genes necessary for MHC I and II molecules (except for β2 microglobulin of MHC I) MHC is 4 million base pairs in length genes on chromosome are polymorphic (have many alleles) →inheritance of these alleles follow typical Mendelian fashion Key points: 4 possible genotypes, can be heterozygous/homozygous for a gene, 25% chance sibling will have same alleles as you MHC genes are codominant combination of alleles are segregated all together as a haplotype (combo of alleles on chromosome) →so alleles of each gene on single copy of chromosome are inherited together each MHC molecule can bind one peptide that fits its structural motif requirements in order to make sure viral peptides are able to bind to MHCs and activate T cells for their destruction, there is a constant turnover of newly synthesized MHC molecules that move to surface

Complement pathways

Classical Mannose-Binding Lectin Alternative

Action of the Complement System

Classical Pathway initiation requires the deposition of IgM or IgG antibody (and also some less important non-Ig proteins like c-reactive protein) on surface of pathogen ∙w/o complement system, antibodies themselves would have no intrinsic killing activity ∙this process endows IgG and IgM with killing capacity C1 is the first component of the complement that binds ∙subsequently, it undergoes a conformational change, and generates an active serine protease that cleaves C4 into C4a (soluble) and C4b (binds to surface of pathogen) ∙C1 also cleaves C2 into C2a and C2b Mannose-Binding Lectin Pathway (MBL) follows same sequence as classical, except it uses MASP-1 and MASP-2 instead of C1 ∙MBL binds to mannose arrays on pathogen's surface and undergoes a conformational change that generates two serine proteases MASP-1/MASP-2, these proteases cleave C4 and C2 for MBL pathway Binding of C2b onto C4b creates C4b2b, the active C3 convertase C4b2b, the C3 convertase, does as its name suggests, converts C3 into C3a and C3b ∙this step can be repeated thousands of times for amplification ∙Finding C3's to cleave is not a problem since C3 is the component of the complement cascade that is at its highest concentration in blood ∙cleavage produces 3 powerful chemotactic factors (C3a, C5a, C4a → which act on vascular epithelium to recruit neutrophils from blood to site of infection so that innate immunity can do its thing) and C3b (an opsonin) ∙Opsonins are molecules that bind antigens and make them a target of the immune system →result is a membrane bound with many C3b handles, allowing receptors on phagocytes (Dendritic cells, b-cells, neutrophils) to capture the C3b bound pathogen If C3b binds to C3 convertase (C4b2b) itself, it generates C5 convertase (C4b2b3b), which makes C5a and C5b ∙cells have complement receptors that recognize all these cleavage products and drive expansion of acute inflammation ∙basically, all cells that are part of the ummune system will be activated by complement proteins and initiate their immune function C5b is deposited onto the pathogen's cell membrane and together with other terminal components of the cascade (C6-9) inserts a ring structure into the membrane Removal of immune complexes Final note: Chemotactic factors C3a and C5a are early promoters of acute inflammation, act to mast cells (granulation) and on tissues around blood vessels (recruit histamine)

Complement Components

Components of the Classical pathway are named with the letter C followed by a number (C1, C4, C2, C3, C5, C6, C7, C8, C9) ∙NOTE: C4 comes before C2 Activation of the complement system leads to a series of serine protease cleavages of inactive zymogens into the activated a and b components ∙a fragments are soluble chemotactic mediators ∙b fragments are larger and bind covalently to the pathogen →Bigger and pathogen Binding ∙C2 is an exception, with larger of the two breakdown products being C2a Mannose-Binding Lectin Pathway has two proteases associated with it, MASP-1 and MASP-2 The components associated with the Alternative Pathway are B and D ∙these components are also broken down into Ba and Bb

Types of Dendritic Cells

Conventional (Licensed) Dendritic Cells ∙full activated DC ∙contain all components necessary to activate naïve T-cells →Pathogen-Recognition Receptors (PRR) →cell-adhesion molecules →co-stimulatory molecules →both type of MHC's Plasmacytoid DC ∙detects viral infection using TLR7, TLR9, and various PRRs ∙secretes IFN-alpha and IFN-beta (more than any cell in the body), which can make cells resistant to viral-infectoin

Routes of Dendritic Cell Processing and Presentation

Dendritic cells engulf and present antigen in a number of ways can use receptor-mediated endocytosis and macropinocytosis to display antigens on MHC II molecules to CD4+ T-cells can also respond to viral infection or viral transfer from another dendritic cell and display antigens on MHC I molecules to CD8+ cells ∙Langerhan's cells were used to describe "viral transfer" method of antigen processing dendritic cells can also use cross-presentation to display antigen on an MHC that it is not normally on (i.e. endocytosed pathogen peptide on MHC I or cytosolic pathogen on MHC II) ∙most common type of cross-presentation is from endocytic to cytosolic pathway

Antigen-Presenting Cells

Dendritic cells, macrophages and B cells can all act as APCs and activate T-cells, but only dendritic cells can activate naïve T-cells ∙this is because dendritic cells are only APCs about to produce signal 2 - ligation to the T-cell costimulatory molecule (CD4 or CD8) allowing T-cell to survive ∙Signal 1 is activation signal to T-cells through ligation of TCR to APCs MHC-peptide complex ∙Signal 3 is the release of cytokines by APC (and sometimes T-cell itself) which drive T-cells to differentiate T-cell activation requires both the activation and survival signals (1 and 2) and they must come from the same APC

IELs

Different types of CD8+ T-cells: ∙α/β CD8+ T-cells look for virally infected mucosal epithelia cells ∙γ/δ CD8+ T-cells look for the expression of stress proteins on the surface of cells or the up-regulation of non-classical MHC molecules, MICA and MICA →these T-cells look to directly ligate these stress proteins so they can kill the epithelial cells expressing them → they do not kill epithelial cells using the MHC-peptide mechanism

Lymphocyte Receptors

Each receptor is composed of heavy chain, light chain and a constant region ~ first 120 AAs in heavy and light chains are variable region, differ from one lymphocyte to the next, allow for huge repertoire of immune cells, region that is solely responsible for binding specificities Heavy chains created through combination of V, D, and J segments ∙one J segment is combined with one D, then J-D combo connected to one V segment, together encodes heavy chain Light chain created solely from J and V segments ∙DO NOT HAVE D SEGMENTS Creation of chains occurs randomly, in an antigen independent matter → nothing is influencing cells on which segments to pick Receptors made up of anti-B-pleated sheets ∙often used to make B-barrels, which is exactly what binding arms of receptors are structured as 3 hypervariable regions have immensely more variability than the other regions ∙regions between them are known as framework regions ∙hypervariable regions tend to match up with loops and turns that are located at the end of the anti-B-pleated sheet, that hold the sheet together →loops and turns are what interact with antigen, function as paratope that binds epitope

Mediator-releasing cells

Eosinophils, basophils and mast cells contain pharmacologically active components stored in granules such as histamine, serotonin, etc., which act on blood vessels and smooth muscle are mostly involved in antiparasite immunity, can also trigger local inflammatory responses

Factors that largely determine immunogenicity

Foreignness →as foreignness increases, immunogenicity increases Molecular Size →in general, the bigger the antigen, the more immunogenic it will be →natural antigens are typically >100kD →antigens that are <10kD are usually weak →→Penicillin is a good example of a small molecule that violates this rule Chemical composition/heterogeneity →the more complex the antigen, the more immunogenic it is, generally →if you increase heterogeneity or compositional complexity, you increase the immunogenicity of the antigen Amino acid composition →increased composition of aromatic AAs increased immunogenicity

Contributions of biological system to immunogenicity

Genotype of Host ∙specificity of MHC molecules varies from person to person →the MHC locus is the most polymorphic of all ∙3 MHC I genes (HLA-A, HLA-B, HLA-C) →two alleles of each gene, 6 total ∙MHC II is more complicated, a person may have between 6-8 MHC II alleles ∙b/c of high degree of polymorphish, certain lpeople lach MHCs that are necessary for presentation or recognition of certain proteins Mode of immunization →depends on route of immunization (intramuscular, subQ, intradermal, peroroal, intranasal, etc.) →dose also matters →→→suboptimal dose does not elicit a response →→→extremely high dose decreases immunogenicity →form matters - particulate angitgens are more potent

Time Frames of Recipient to Donor Rejection

Hyperacute rejection Acute rejection Chronic Rejection

Lymphocytes test their receptors

IF receptors react with host cells, they are given a few chances to edit the receptors to make them not react with host cells →if they still react with host cells, they are destroyed; if not, they are allowed to leave bone marrow/thymus and proceed to secondary lymphocytes if they bind antigen in body, they undergo clonal expansion and that is where you get population of plasma effector cells and memory cells from

Important Pro-Inflammatory Cytokines

IL-1 - activates vascular endothelium TNF-α - activates vascular endothelium and increases vascular permeability IL-6 - lymphocyte activation and increased antibody production All three previous cytokines have systemic effects that cause whole body response or acute phase response, acting on: ∙Hypothalamus - increase body temp, causing fever ∙Liver - increase release of antimicrobial factors ∙Bone Marrow - generate more polymorphoneculear factors →shift to the left from increase in immature polymorphonuclear leukocytes, shows active inflammation

Other important cytokines

IL-8 - chemotactic factor, recruits neutrophils, basophils and T-cell to infection site IL-12 - activates NK cells

T-cells must show weak binding to MHC molecules and self peptides

If it instead shows slightly increased binding towards both, it is not destroyed → it becomes regulatory T-cell

How do we get IgA into external secretions?

IgA is secreted as a dimer with its J chain from a plasma cell sitting just below epithelial cell At basolateral surface of epithelial cell, there is a Poly-If receptor, which binds polymeric Igs, (i.e IgA and IgM), it binds covalently to IgA Poly-If receptor-bound IgA is taken up by endocytosis, transported in a vacuole to apical surface of the endothelial cell where it fuses with the apical surface, and liberated into external secretions When the dimer is released via proteolysis of the Poly-If receptor, a little portion of the receptor is released with it into the external secretions ∙portion of receptor released is the SC component of SIgA on its way though epithelia cell, Poly-Ig receptor-bound IgA can intercept Ags that have penetrated epithelial cell ∙alternatively, IgA in the lamina propria (that has yet to bind to the Poly-If receptor) can pick up Ag that have made it through the endothelial cell into the lamina propria and transport them out of the lamina propria

IgD

IgD has a limited role in the immune response really is just a marker of the mature B cell

Type III response: Immune complex reactions

IgG antibodies also mediate Type III reactions however, Type III reactions result of formation of immune complexes, which can be generalized (systemic) or localized drugs reactions, such as penicillin, can produce Type III response type of lattices that are formed in interactions b/w an antibody and an antigen determines whether immune complexes will be formed that lead to type III reactions Equivalence is determined as the state that occurs when there is an efficient 3-dimensional force lattice formation b/w antigens and antibodies under normal circumstances, immune complexes at this state are removed by RBCs that bind to complement protein C3b that is on immune complex, which delivers it to liver or spleen to be processed ∙when there is antigen excess, immune complexes are difficult to remove from blood

Type II responses are IgG mediated and occur on surfaces of cells

IgG antibodies cause destruction of cells, normally RBCs (hemolytic anemia) or platelets (thrombocytopenia) by binding to surface of cell types of antigens that produce type II reaction are drugs such as penicillin ∙unknown why minority of people make antibodies to these drugs only priming occurs first time antigen is presented to sensitive individual ∙in subsequent presentation, inflammatory response will occur Damage can be inflicted in three ways: ∙complement mediated cell lysis ∙phagocytosis and clearance by macrophages ∙antibody dependent cell mediated cytotoxicity (ADCC) which is inflicted by NK cells →NK cells can respond to IgG's because they contain Fcγ receptor

Summary of Ig Classes

IgG has highest serum concentration & half life IgA has highest rate of synthesis

Type I Immune Tissue Injury

Immediate hypersensitivity reactions → IgE mediated allergic diseases Theories on how allergic responses are produced in atopic individuals: ∙Sensitization to an allergen involves class switching to IgE production on first contact with allergen →Two models for sensitization 1 - the allergen enters the mucosa and is taken up by dendritic cell, Th2 priming occurs in lymph node where DC cell acts on naïve T cells, then Th2 cell acts on plasma cells in lymph node to switch to IgE production, plasma cell travels back to mucosa and produces IgE's against allergen, which bind to Fc receptor mast cells, when allergens bind to IgE and crosslinks two of them it causes degranulation to occur in mast cell 2 - allergen is complexed with LPS, a molecule that can ligate TLRs, ligation of TLRs delivers signal to dendritic cell that is going to activate T cell to produce cytokines, which eventually leads to class switching of IgE production in B cell ∙Resting state to active state →In resting state, immunosuppressive cytokines are normally expressed that upregulate regulatory T cells, whereas other T cells are down regulated →in an allergic individual, when allergens act on TLR of epithelial cells, pro-inflammatory molecules are released from them that activate DCs →→→activated DC then polarizes naïve T cells into becoming Th2 cells, which can release cytokines that can help B cells class switch to IgE ∙Signals required for class switching →Release of cytokines for Th2 cells is main way class switching occurs →mast cells and basophils can also amplify IgE production →when these molecules are IgE bound and antigen cross links 2 IgE molecules, production of IL-4 occurs, which can act on plasma cells that are nearby the site of inflammation →co-stimulatory molecules such as CD40 (B cell) and CD40L ligation (T cell) are also required for class switching to occur in both cases ∙Signals favoring Th2 polarization →one signal that favors Th2 T cell production is interaction b/w APC and naïve T cell →other signals are low antigen dose, route of presentation (skin and mucosa) and cytokines →→→IL-4 drives class switching to IgE and smooth muscle contraction →→→IL-5 promotes eosinophils differentiation →→→IL-13 promotes smooth muscle contraction

Immunoglobulin basics

Immunoglobulins (Ig or Ab) are the primary components of the humoral immune response Five classes of Ig's (IgM, IgD, IgA, IgG, IgE) that ca be synthesized in roughly this order by maturing and activating B cells ∙classes also known as isotypes ∙classes correspond to differences in heavy chains of constant region Ig's can be membrane bound (IgD, some IgM) or secreted (IgA, IgG, IgE, some IgM) into the "humors" →humors = blood, tears, saliva, mucous Ig's are antigen specific, primary function is to directly bind to antigens and lead to their neutralization, removal or destruction

Locations of lymphoid cells in mucosal immune system

In lamina propria: ∙we have a very heterogenous mixture of cells that is dominated by CD4+ T-cells, but you also have mast cells, eosinophils, plasma B-cells, etc. In the epithelium: ∙We have CD8+ T-cells that are either α/β or γ/δ, along with various forms of CD8+ co-receptors ∙intraepithelial lymphocytes also have a major role in the maintenance of the integrity of the barrier epithelium

Difference between innate and adaptive immune system in receptor expression

Innate system has al of its receptors coded for in genome and express the same receptors transcribed from same genetic material time after time adaptive system takes different segments of genes and combines them in novel sequences to develop new receptors on ever cell ∙process is completely random ∙means that lymphocytes may develop receptors that bind cells of host → bone marrow and thymus, as organs that produce these lymphocytes, make sure any lymphocytes that identify host cells do not hurt body

Class Switching & B-cells

LPS - potent immunostimulatory molecule Adding LPS to mouse with naïve B-cells turned on all B-Cells, induced all cells to proliferate ∙when different cytokines added after LPS, able to see that different cytokines induced switching to different isotypes of immunoglobulins ∙IL-4 switched to subtype IgG and IgG ∙TGF-beta switched to IgG and IgA cytokines have ability to stimulate switching to certain types of immunoglobulins, but at same time, inhibited other types different classes of immunoglobulins attach to different cell types with differing affinities ∙since only constant region is changed

Leukocytes vs. Lymphocytes

Leukocytes = WBCs ∙granulocytes →neutrophils →basophils →eosinophils ∙agranulocytes →Monocytes →Lymphocytes Lymphocytes = type of leukocyte ∙B cells ∙T cells ∙NK cells

Variation in Solid Organ Transport

Liver has low HLA expression, is more resistant to rejection Heart transplant there is generally no time to do all diagnostic testing, so clinicians usually try to determine if patient has been sensitized to MHC expressed by heart donor Also exist immunologically privileged sites which are great for a transplant, like cornea ∙matching isn't issue here

Secondary Lymphoid Tissues of Importance

Lymph Nodes Spleen Peyer's patches

MHC I

MHC I has 3 types: HLA-A, HLA-B, HLA-C interact with CD8 cytotoxic T cells are displayed on ALL nucleated cells

MHC picking up peptides

MHC I: an N-terminal signal (leader) polypeptide targest the 2 polypeptide chains into ER where they pick up one out of 2000 different peptides that are about 10 AAs long, which stabilizes MHC molecule ∙MHC I then goes to golgi, finally displays its peptide on cell surface ∙peptide could be fragments of normal cellular protein degradation, could be fragments of pathogen MHCII: Class II picks up peptides that are outside of cell through endocytic pathway ∙binding to invariant chain in ER prevents MHC II molecules from binding to peptides in ER like MHC I does ∙invariant chain is released in endocytic pathway

MHC II

MHC II has three types: HLA-DR, HLA-DQ, HLA-DP interact with CD4 helper T cells displayed on cells of immune system (B lymphocytes, macrophages, dendritic cells, macrophages, activated T cells) inducible on other types of cells (i.e. endothelial cells) under certain circumstances

4 Types of Tissue Injury

Main differences: ∙Type I, II and III are all antibody mediated ∙Type IV is cell mediated ∙additionally, Type I is mediated by IgE antibodies, Type II and III mediated by IgG Focusing on IgG mediated responses: ∙Type II occurs on cell membranes ∙Type III occurs in places were there are immune complexes (tissue fluid, blood, etc)

B Cell Maturation

Mature B cell (expressing IgM and IgD) is only activated upon its exposure to its respective antigen, and that activation is prerequisite in the production of IgA, IgG and IgE

APC's Activate T-cells in Lymph Nodes

Mature T-cells from thymus enter circulation and migrate between blood and lymph, weakly binding ("sampling") MHC-peptide complexes on cellular surfaces in search for their cognate antigen if MHC holds a peptide that is not T-cells cognate peptide (self or non-self) the T-cell unbinds and continues to circulate T cells are lured into lymph nodes by cytokines ∙enter by crossing high-endothelial venules (HEV) just like NK cells, and locate to paracortical areas Dendritic cells, most significant T-cell activating APC, starts life as phagocyte ∙once it takes up antigen, it becomes an APC by displaying the MHC peptide complex on its surface, as well as upregulating its cytokine receptors, which allows it to respond to cytokines and migrate to lymph nodes Dendritic cells congregate in the paracortical (T-cell area_ and cortical regions of lymph nodes Macrophages locate all over lymph node, but are concentrated in medulla and cortical sinuses B-cells locate to B-cell areas ∙have tens of thousands of Igs on surface, which makes B-cells the best APC for recognizing soluble antigen, which they bring in using endocytosis and express the antigen on MHC II once T-cell and APC are both in lymph node, T-cell can identify its cognate MHC-peptide complex on APC and become activated to an effector T-cell, differentiate and re-enter blood circulation

Overall cycle for B-cell

Naïve B-cell → lymph node → encounter antigen (delivered by SCS macrophage or follicular dendritic cell) → encounter T-cell → undergoes cell division → some form primary focus (IgM) = early immune response, rest go into follicle and form germinal center → undergo somatic hypermutation and class switching (with help from T-cell) → terminally differentiate into plasma cells → back to bone marrow, produce antibodies for secretion

Affinity Maturation

Over time, hypermutation fine tunes our affinity for specific antigens process is known as affinity maturation

Common PAMPs

PAMPs often seen in bacteria include peptidoglycan of the cell wall, unmethylated CpG DNA of prokaryotic nucleic acids, flagellin and pillin proteins from bacterial flagella and pilli, and lipopolysaccharide and lipoteichoic acid from gram negative and gram positive bacteria PRRs also recognize mannose and zymosan (a complex carb) of fungal cells walls and double stranded RNA from viruses certain self molecules are similar to PAMPS, these molecules are called DAMPS/alarmins and they can sometimes cause inflammatory response ∙example is uric acid

Regulation of the complement system

Pathway activation and subsequent MAC formation can occur on our own cells would cause immune system complications, if not for tight regulation of each complement pathway step

Vaccine development

Peyer's patch seeds back to the small intestine, but also seed the mammary glands, particularly during pregnancy ∙Abs in the molk wouldn't just recognize Ags in the gut, but also Ags found in the mouth because these Ags would be swallowed later and picked up in gut - the way a mother can provide passive protection to child Nasal associated lymphoid tissues (NALT): seed back to origin, but also to urogenital tract ∙for VD vaccines, rather than having to induce immunization by local application of Ag, we can immunize via the nose and produce high levels of Abs in urogenital tract

Functions of the Complement System

Phagocytosis by opsonization ∙coats the pathogen with a molecule (think b) that will facilitate uptake of that pathogen by phagocytic cells Promote inflammation ∙recruits inflammatory cells by liberated soluble chemotactic factors (think a) Kills microorganisms ∙forms a membrane attack complex that inserts pores into the pathogen's membrane, leading to death by osmotic lysis (can also kill host cells → pathological) B cell activation & maturation Immune complex removal ∙solubilizes and facilitates the removal of antibody attached to foreign molecule complex, filtered by kidney T cell activity modulation/maturation

IgA Deficiency

Q: If IgA is so important, why can people with IgA deficiency (most common Ig deficiency) stay entirely healthy? A: IgM is another polymeric Ig that can bind to Poly-Ig receptor, therefore, many patients with selective deficiency of IgA compensate by secreting IgM (secretory IgM) Q: Why don't people who secrete IgA also secrete IgM? A: IgM associates non-covalently with the receptor, while IgA associates covalently

Signal 1 of B-cell activation

Signal 1 comes from the pathogen 2 different types ∙Thymus-dependent antigen (TD) →require T-cell help, usually protein antigens (majority of antigens) ∙Thymus-independent antigen (TI) →do not require T-cell help, usually has a repeating subunit structure, polysaccharides such as pneumococcal bacterial capsules Both types of antigens are required to cross-linked BCRs to activate signal 1 ∙if antigen is multivalent (multiple copies of one epitope), there is a greater chance the BCR binds to same epitope and cross-link, then B-cell phagocytoses bacterium degrades it, and presents it on MHC class II

Important signaling receptors in local infection

Signaling Pattern-Recognition Receptors ∙present on cells of immune system (especially phagocytes) as well as somatic cells, receptors allow cell to determine the type, size, and infectious dose of the pathogen by binding to PAMPS, this family includes both transmembrane and cytosolic receptors Toll-like receptors ∙transmembrane PRR receptors that recognize PAMP's at the cell's surface and can therefore sense extracellular organisms, most often found in eukocytes and endothelium but can also be found in endosomes/phagosomes Nod-Like Receptors (NLR) ∙family of cytosolic receptors that can recognize intracellular PAMPs, such as viral components Endocytic Pattern Recognition Receptors (Endocytic PRRs) ∙can be t ransmembrane receptors that act like handles and bind to surface of pathogens to faciliate their uptake by phagocytosis, can also be found in soluble state in plasma and tissue fluid and faciliate endocyotsis ∙also included in endocytic PRR family are complement receptors Membrane endocytic PRRs ∙C-Type Lectin Family - recognize sugar moieties and glycoproteins on pathogens ∙Scavenger Receptors - recognize negatively charged molecules, lipoproteins, and hydrophobic lipid-like materials ∙N-Formyl Met Receptors - bind N-terminal of bacterial proteins Soluble Endocytic PRRs ∙Mannose Binding Lectin - binds mannose of surface of fungi and bacterial capsules ∙C-Reactive Protein - binds phosphorylcholine and related molecules on surface of bacteria ∙Lung Surfactant Proteins - have lectin-like specificity such a limited amount of PRR with low specificity can recognize mant different pathogens and generate a diverse array of response through cooperative interactions between several PRRs to create homo and heterodimers as well as rely on adaptor molecules and use of different signal transduction pathways General outcomes of PRR ligation to its PAMP ∙release of pro-inflammatory cytoknes ∙release of chemokines ∙upregulation and expression of MHC I and MHC II ∙upregulation and expression of co-stimulatory molecules ∙Upregulation and expression of cell adhesion molecules

Location of B-cell activation

Spleen ∙Paracortex →T-cell seek dendritic cells to be activated by antigen →once activated, they express chemokines that direct them towards the follicle to encounter B-cells ∙Follicle →B-cells are activated by antigen →once activated, they also express chemokines and migrate towards boundary of follicle and T-cell area ∙activated T-cells and B-cells interact in junction of paracortex and B-cell follicle →some B-cells go to medulla and differentiate into plasma cells to produce IgM as first line of defense (primary focus) →other B-cells activated by T-cells proliferate and form a germinal center in lymphoid tissue o this is where isotype (class) switching and somatic hypermutation occurs, to compete for antigen among activated B-cells Lymph Node ∙lymph node has afferent lymphatic vessel so any antigen that was previously opsonized, will enter the lymph node and into subcapsular sinus (SCS) ∙SCS is fully equipped to tackle antigens →has SCS specific macrophages that will capture antigen, but will not phagocytize it →they preserve antigens and display them on surface for B-cells, or can be transferred to resident follicular dendritic cells ∙Resident follicular dendritic cells are also present in spleen, similar ot SCS macrophage in sense that they stay and display antigen for B-cell →both have IgG receptor and complement receptor →antigen complexes resemble little beads that lie on dendritic cell waiting for B-cell to come pick them up

Examples of autoimmune diseases

Systemic Lupus Erythematosus (SLE) Grave's disease Type 1 diabetes Myasthenia Gravis

TH-cells (Helper/CD4+) and Treg cell differentiation

T-cell differentiates into each specific class through variation of signal 3 ∙APCs secrete certain cytokines that induce specific transciption factors ∙Note: dendritic cells do NOT produce IL-4, which is required for production of Th2-cells →thought that mast cells, eosinophils and basophils produce this signal ∙what dictates what cytokines are released is which pattern-recognition receptors (PRRs) are ligated by APC ∙Treg-cells are often produced when a CD4+ T-cell receives weak-stimulation from an APC

Cellular Immunity is a 3 part interaction

TCR - peptide - MHC T cell cannot see pathogen on its own without it being presented by MHC

T Cell Receptor (TCR)

TCR is made up of α and β chains held together by disulfide bonds, is always bond to T cell surface complexed with CD3, CD4, CD8 because it is not very good at signaling on its own with its short cytoplasmic tail →this is because T cell has more of a surveillance function, and affinity for TCR for its MHC-peptide is weak TCR is specific for a particular Ag in a complimentary fit, just like how MHC and Ig are specific for a peptide it has dual affinity for both antigenic peptide and MHC T cell can bind self or non-self antigens →autoimmunity will result if T cell is not able to ignore self-antigen carrying MHC first 100 AAs from N-terminal of α and β chains make up variable region of antigen-MHC recognition site Gene rearrangement allows TCR to recognize large repertoire of antigenic fragments, despite the fact that it can only recognize a limited set of self MHC molecules →random formation of different TCRs from a wide library of TCRs is very similar to synthesis of Ig gene rearrangement in Igs →exposure to certain Ag/MHC will activate matching TCR and proliferation of those T cells TCR α chain organization encodes a signaling peptide/leader, variable region, constant region domain, segment next to cell surface, transmembrane and cytoplasmic regions of the polypeptide →germline organization of cell that is not a T cell yet includes 50 V segments, 70 J segments, and 1 segment →1 V segment is then spliced next to a J segment permanently and randomly to make a working variable region as the stem cell matures into T cell β chain does same organization →germline organization of 57 V, 2 D, 13 J and 2 C segments excises parts of DNA to become a variable region consisting of 1 V, 1 D, 1 J hypervariable loops at V-J, V-D-J junctions of α and β chain respectively generate different TCR binding sites →NO SOMATIC HYPERMUTATION

T cell vs. B Cell Epitopes

TCR's only bind linear epitopes and MHC ∙linear peptide epitope is derived from protein antigen that is taken up by APC, processed and presented on MHC to TCR → TCR is MHC restricted ∙length of linear peptide is usually 15-22 AAs, commonly derived from hydrophobic sequence ∙MHC I peptides are usually shorter than MHC II peptides b/c of structural difference sin the MHC molecules BCR's (antibodies) can be directed against nearly any type of molecule imaginable if conditions are right ∙antibodies can bind conformational epitopes like tertiary structure of protein antigens →if antigen's conformation is destroyed by denaturation, conformational epitopes are also destroyed and antibody will no longer bind ∙antibody may also be raised against a linear epitope →if linear epitope present on surface of protein, denaturing protein will not have effect on epitope binding →if antibody is raised against internal linear epitope, protein will need to be denatured before epitode is accessible to antibody ∙proteolytic processing can create new epitopes (neoepitopes) ∙adsorption = when a protein binds to a surface →when a protein binds to a surface, conformation changes can lead to formation of cryptic neoepitopes →cryptic neoepitopes are epitopes that are hidden and then exposed as result of conformational changes, →example is HIV, when it binds to target CD4+ T lymphocyte, virus only displays certain epitopes at time it binds, making it very difficult for researchers to develop neutralizing antibodies against them

The Three Pathways of the Complement System

The Classical Pathway is the ONLY one that is acquired and thereby initiated by antibody participation, specifically IgG or IgM ∙other two pathways do not require presence of antibodies Two of pathways, MBL and Alternative, are innate and don't even require presence of any antibodies →activated immediately upon exposure to infection and begin their attack All 3 pathways utilize the same TWO amplications steps: C3 convertase C4bC2b(a) and C5 convertase C4b2b3b ∙alternate pathway slightly differs in that it uses a functionally similar, but structurally different C3bBb as its C3 convertase Upon generation of the C3 convertase, the three pathways converge into a common pathway until termination

Leukocyte Formation

The common lymphoid lineage gives rise to three types of cells: ∙B lymphocytes, which mature in bone marrow and are involved in humoral immunity (gives rise to plasma cells which make antibodies and memory B cells) ∙T lymphocytes, which mature in thymus and are involved in cell-mediated immunity ∙Natural Killer (NK) lymphocytes, which are involved in innate immunitiy, and serve as defense against intracellular infections (viral infections) →do not have antigen-specific receptors, nonspecific Second major branch of immune cells is derived from common myeloid lineage, which gives rise to granulocytes and megakaryocytes blood holds an average of 5,000-10,000 leukocytes/uL in circulation →most abundant myeloid cell is neutrophil (40-70%)

Thymocytes at different developmental stages are found in distinct parts of the thymus

The earliest precursor thymocytes enter the thymus from the bloodstream via venules near the cortico-medullary junction. Ligands that interact with the receptor Notch1 are expressed in the thymus and act on the immigrant cells to commit them to the T-cell lineage. As these cells differentiate through the early CD4-CD8- double-negative (DN) stages described in the text, they migrate through the cortico-medullary junction and to the outer cortex. DN3 cells reside near the subcapsular region. As the progenitor matures further to the CD4+CD8+ double-positive stage, it migrates back through the cortex. Finally, the medulla contains only mature single-positive T cells, which eventually leave the thymus.

Hygiene hypothesis

Theory that individuals that have high genetic susceptibility and live in a 'hygienic environment' (low exposure to microorganisms and illnesses) are most likely to be atopic or allergic individuals who live in less hygienic environment and have low genetic susceptibility are non-allergic or non-atopic as result of early exposure to variety of microorganisms during infancy and childhood

Immunoglobulins generally act as monomers

There are a few exceptions: IgA & serum IgM →both are also held together by a J chain IgA is a dimer linked by a secretory component serum IgM is a pentamer, has 10 binding sites (2 per monomer), so it is extremely useful in agglutination of antigens ∙structure of IgM also effective in activating C1, an initiator of the Classical Complement Cascade ∙C1 requires 2 adjacent Igs for activation, so serum IgM's five monomers are always in position for this

MHC Structure

Things to note about MHC I: ∙α is heavy chain (not like in Ig, different kind of cell), there is a smaller β2 microglobulin ∙only α chain is inserted into cell membrane ∙know which ones are α1, α2, and α3

Importance of Thymus

Thymus plays important role in T-cell development as cells from bone marrow arrive, they are triple negatives, they show neither CD-3 (marker of T-cells, CD-4 or CD-8 ∙thymus signals for expression of CD-3, at which point cells are characterized and identified as T-cells ∙about 20% are gamma-delta cells and leave thymus early to carry out other roles in mucosal membranes ∙other 80% are alpha-beta cells and are considered double negative →then become double positive (expressing both CD-4 and CD-8 receptors), then they are selected for and at the end show only CD-3 and either CD-4 or CD-8 Thymus and bone marrow interact closely ∙need both to work perfectly for T-cell development ∙bone marrow is responsible for rearranging genes to make T-cell receptors ∙thymus responsible for making T-cells

Thymus & T-Cells

Thymus sits above heart, is site for thymocyte (T-cell) development ∙can be understood to be sieve of cortical epithelial cells surrounding a medulla (B-cells and dendritic cells) Immature cells migrate from bone marrow with T-cell receptors developed from gene segments ∙T-cells start in cortex, migrate through epithelial cells to reach medulla ∙role of cortical epithelial cells is to test developing T-cells to make sure they can recognize host MHC I and II molecules - POSITIVE SELECTION →any T cells that do not recognize MHCs are quickly disposed of ∙take identity of CD-4 or CD-8 at this stage →begin with both CD-4 and CD-8 receptors, →if first ligation happens with MHC 2 molecule, it loses CD-8 receptors and commits to becoming CD-4, and vice versa T-cells also get a chance to undergo receptor editing

Signal 2 of B-cell activation

Thymus-dependent: ∙The MHC II+ complex then interacts with specific helper T cell →this helper T-cell has already been activated by a dendritic cell that presented the same MHC II+ peptide sequence ∙once helper T-cell binds with MHC II+ peptide, it upregulates CD40 (on B-cell) and CD40L (on T-cell) and they interact becoming signal 2 ∙then helper T-cell can secrete cytokines to induce somatic hypermutation and class switching ∙B-cell will differentiate into ether a plasma OR a memory cell →when a B-cell undergoes class switching the specificity of the antibodies are identical → only the constant domains are changed ∙Cognate interactions: the interaction b/w T-cell and B-cell is extremely close, its actually called an "immunological synapse" →Advantage b/c there is little activation of any bystanders cells, which further increases the specificity of the adaptive immune response Thymus-independent ∙signal 2 can be delivered multiple ways without involvement of T-cells →repeating structure on bacterium binds enough receptors to not only trigger signal 1, but also signal 2 (these are TI-1 class of antigens o at high concentrations they can actually induce proliferation of multiple clones of B-cells → a non-specific antibody response o at low concentrations, they will select for the antigen specific response of one clone of B-cells ∙DNA, or lipopolysaccharides (LPS) from bacterium binds to Toll-like receptors, delivering signal 2 o after activation with signal 2, without a T-cell the necessary cytokines for class switching are missing so the B-cells can only secrete IgM, they also do not undergo somatic hypermutation →→→without T-cell help, ONLY CAN MAKE IgM o dendritic cells can release a cytokine (BAFF) that can not only activate the B-cell, but also induce class switching for B-cell to secrete IgG (these are the TI-2 class of antigens) Remember for TD antigen activation that helper T-cells have already been activated by dendritic cell ∙therefore the helper T-cell - dendritic cell interaction via MHC II + peptide needs to be the same interaction that the helper: T-cell: B-cell has to guarantee specificity

Types of hemolytic anemias

Transfusion reactions - most common way hemolytic anemia can occur in Type II reaction ∙IgG's recognize foreign ABO groups on RB membrane and bind to them, activating complement system, which leads to destruction of foreign RBCs Drug-induced hemolytic anemia - occurs when drug acts as hapten molecule and RBC or platelet acts as carrier ∙IgG antibodies bind the surface of the cell, leading to activation of complement Erythroblastosis fetalis - hemolytic anemia that occurs in fetus ∙occurs when mother is carrying Rh+ fetus but mother is Rh- ∙after parurition, there is some leakage of fetal blood cells into maternal circulation, which prime maternal circulation ∙mother produces anti-Rh IgG antibodies ∙if there is a second pregnancy and the fetus is Rh+, the anti-Rh antbodies will cross the placenta and lyse fetal RBCs →remember that IgG antibodies can cross placenta ∙situation can by prevented with rhogram, an antibody to the rhesus antigen →rhogram acts like sponge → binds to rhesus before priming occurs in maternal circulation →can only work if priming has not occurred yet in mother → mother would have to take it after she has her first Rh+ child ∙to determine if there is rhesus incompatibility, a Coomb's test is performed →in indirect Coomb's test, goal is to determine whether the mother has anti-Rh IgG antibodies →→→mother's serum is isolated and tested with Rh+ RBCs to determine whether the mother has IgG's against it →→→Rabbit anti-human antibody recognizes the human IgG and causes agglutination, which allows for visualization →in direct Coomb's test, fetal RBCs are taken to determine whether they are Rh+ →→→mother's antibodies are used to see whether binding occurs →→→rabbit antibody is again used to visualize

Heavy Chain Gene Rearrangement

V, J D encode variable region →approximately 51 V segments, 6 J segments, 27 D segments variable exons followed by constant region exons of heavy chain that define its class in long line of C segments, only furthest upstream (most to left) will be expressed ∙Cm (mu) and Cd (delta) responsible for IgM and IgD isotypes, come first ∙remaining classes follow afterwards (gamma, alpha, epsilon)

Inductive Sites

Waldeyer's ring - the protective roing of moth and nose ∙consists of pharyngeal tonsils, adenoid tonsils, and lingual tonsils, which are all IgA inductive sites ∙**In the natural portals of entry - nose, mouth, genitourinary tract - there are protective rings of lymphoid tissue Appendix - inductive site in the large bowel Lymphoid aggregates - inductive site in the rectum Peyer's patches - inductive sites in the small bowel

Understanding Minor Histocompatibility Complex Issues in Transplantation

a patient's APCs can pick up graft fragments from a transplant ∙in these fragments may exist proteins that seem suspicious to recipient ∙like if a male kidney went to female recipient

Epitope

a site on an antigen recognized by an antigen receptor (or free antibody) T cell epitope is a short linear peptide derived from a protein antigen, it binds to an MHC molecule and is recognized by a particular T-cell B cell epitopes are antigenic determinants recognized by B-cells and are typically structural motifs on the surface of an antigen

TCR Clinical Correlates

aberrant rearrangements inplicated in some malignancies: ∙incorrect DNA rearrangement can lead to inactive genes that can ultimately result in a tumor ∙virally infected tumor cells can lose HLA I expression rearrangement monitored during immune reconstitution after bone marrow transplant ∙screen TCR gene and AB gene as marker for diagnosis since rearrangement is unique ∙can determine if T cell proliferation is from single T cell (monoclonal response) or from many T cells (polyclonal) ∙can stage T cell tumors as acute or chronic depending on whether TCR locus is rearranged or not

Immunoglobulin Specificity

ability of an antibody to react with a molecule can be diminished by changing position of a single functional group antibodies are not as good at distinguishing between different functional groups at same position

Allorecognition

ability of an individual organism to distinguish itself from someone else can result in graft rejection

IgE

activates eosinophils, basophils and mast cells plays central role in protecting body from parasites and in body's allergic response effects are secondary to Fc receptor binding Eosinophils, once bound with IgE, degranulate and release toxic substances which damage and kill parasite Mast cells and basophils release histamine when triggered by IgE →histamine triggers inflammatory response

Overview of B-Cell Mediated Immunity

adaptive humoral response is mediated by B-cells, which develop into plasma cells and secrete antibodies Plasma cells are terminally differentiated B cells that are antibody secreting factories antibodies can go further and activate complement, neutralize and opsonize bacteria

Losing a Graft

after losing graft you now have developed sensitivity to new set of MHC alleles, making a new transplant a tougher process match-wise in HPC transplant, can try and use less immunogenic tissue like umbilical cord blood ∙blood is very rich in stem cells that are more mature than adult bone more, which is good b/c there is a lesser chance of immune recognition ∙however, very few stem cells in umbilical cord relative to what exists in an adult, so repopulating an adult may prove difficult In future, would be great if we could induce specific tolerance ∙would be holy grail of transplantation ∙basically would be procedure that would make graft look and act like self

Isotype Switching

aka class switching following activation of a B lymphocyte, a process known as isotype switching can occur process is mediated by cytokines, is a way for body to adapt Ig's produced to situation at hand switching sequences allows DNA to clip out intervening segments and move corresponding C exon next to VDJ complex ∙this is why order of C regions is important ∙remember MD GAE ∙remember that Cm and Cd (for IgM and IgD) come first Isotype switching is a one-way street: although further downstream switches can occur (i.e. IgA → IgE), an IgA cannot go back to IgG →b/c information b/w regions gets destroyed when changing to a new isotype

Hypervariable regions (HVR)

aka complementarity determining regions (CDR) they are what give variable regions their variability ∙are the components of variable regions that fold to actually bind with antigen ∙three per variable region, remaining portions are unchanging

Hematopoietic Cells & Cell Types

all circulating cells of immune system and blood arise from pluripotent hematopoietic stem cells in the bone marrow bone marrow stromal cells provide cytokines aand other growth factors which control the differentiation of the hematopoietic stem cell into immune cells, tissues, and components at same time, chemokines act on progenitor cells to keep them within bone marrow pluripotent hematopoietic stem cell can end up producing leukocytes (WBCs), granulocytes, erythrocytes (RBCs), and magekaryocytes (platelet-producing cells) important to note that RBCs and platelets serve important functions in clotting and circulating all of the immune cells

Isotype Switching and Affinity

all class switching change is in constant region, therefore, variable region's antigen binding specificity is unchanged Effector functions are changed, however

Immune tissue responses are mediated by Gell & Coombs type II, III and IV

all hypersensitivity reactions involve IgG and are transferred across placenta child will be born with disease, but once it is able to produce its own antibodies and maternal IgG dissipates, child will no longer carry disorder

AIRE genes

allows bone marrow and thymus to express genes that are not expressed there normally (i.e. insulin) to test lymphocytes

Valence and determinance

antigens that have a single epitope are called unideterminant univalent if antigen has multiple epitopes of the same type it is unideterminant multivalent if antigen has many different types of epitopes it is termed multideterminant multivalent

Complement System Deficiency

any deficiency in compliment proteins results in increased susceptibility to extracellular bacterial infections deficiencies in the earlier steps (those leading up to generation of C3b) give rise to immune complex disease ∙immune complex disease = lupus-like disease where one is unable to remove immune complexes ∙should make sense, since removal requires C3b Deficiencies in terminal components of complement (C6-C9) renders individuals susceptible to recurrent nisserial infections it is possible to have deficiencies in regulatory proteins, which would lead to uncontrolled activation of the complement system and hereditary angioedema ∙example = defect in C1 inhibitor ∙results in uncontrolled activation of complement cascades, which leads to extreme inflammation, vascular dilation, and recruitment of immune cells to the area ∙life threatening condition b/c swelling may obstruct airway and lead to death by asphyxiation

Antigen

any molecule that can bind specifically to an antibody (i.e. B-Cell receptor (BCR)) or generate peptide fragments that are recognized by a T-cell receptor (TCR)

Immunogen

any molecule that, on its own, is able to elicit an adaptive immune response (i.e. antibody production against it) de nobo on injection into a person or animal

Cryptic Epitopes

arise from any of several mechanisms that result in inefficient antigen processing and presentation when T cell is being positively and negatively selected for in thymus, there are many peptides being generated with only major ones being expressed on MHC within thymus ∙allows T cells that are reactive to minor peptides to move to periphery where they can react with peptide if presented by APC while under appropriate activating inflammatory conditions Clinical: ∙Goodpasture's syndrome →presentation with kidney failure and bleeding in lungs

Immune Exclusion

as a prerequisite for a pathogen to enter the body, it must adhere to the barrier epithelia → most effective means of defense is to never let bacteria adhere in the first place = IMMUNE EXCLUSION ∙exclude bacteria from even penetrating microbiota

Chemokines

attract immune cells to various locations in body serve as body's GPS system

Basics of autoimmunity

autoimmunity is a breakdown in immune regulation that leaves the immune system unable to differentiate self from non-self large genetic influence in these breakdowns, but environmental and infectious agaents can play a role overall affect can be organ-specific or systemic unfortunately, females are disproportionately affected

Epitope Spreading

based on linked recognition basically, autoreactive T cells can be bad influences and drive production of autoantibodies of different specificities ∙i.e. H1 (histone complex) specific T cell activiates H1 specific B cells wen B cells process H1-containing nucleosomes that are floating around following cellular damage →here the B cell binds directly to histone molecule, processes and presents the peptide →conversely, same T cell can activate a different B cell specific for DNA stretches →→→B cell endocytoses the nucleosome that also contains an H1 complex and presents the H1 complex the T cell wants to see, thereby allowing for its activation to plasma cell producing anti-DNA stretch antibody →→→are now two or more different plasma cells making antibodies for varying specificities →→→allows reaction to continue to expand again, leading to chronic nature of autoimmune disorders →→→allows T cell to be master to many servants

Hyperacute Rejection

body starts rejecting graft while on OR table means you've already been sensitized to human tissue of this type and have antibodies in you that are ready to attack it immune response begins immediately and organ is destroyed in a short time

Progenitor cells give rise to lymphocytes

both B and T cells progenitor cells undergo cell division and each progeny randomly pieces together gene segments to develop different receptors with different specificities Important: one lymphocyte may have thousands of receptors but each receptor is identical and has one specificity; receptors of different lymphocytes are not identical and have different specificities bottom line: one cell, one receptor expressed all over, one specificity

Haptens

by coating large self proteins with foreign molecules, you have potentially created immunogenic antigen hapten = small organic molecule of simple structure that does not provoke antibodies when injected, however, antibodies can be raised against them if they are covalently attached to a protein carrier →important for vaccine development →in vaccines, protein carrier is from different animal and thus recipient will generate antibodies against carrier

Functions of Normal Microbiota

commensal biota = non-harmful, symbiotic bacteria normally found in bodies and on skin that help defend us against colonization of more pathogenic invaders microbiota are critical for health of immune system ∙when broad spectrum antibiotics are taken, commensal biota is decminated and fungi or anti-biotic resistant microorganisms will take their place and cause pathology normal resident microbiota prevent colonization of more harmful competitors by competing for nutrients, competing for receptors (which block other pathogens from adhering), and producing antagonists (such as their own antibiotics, H2O2, etc) microbiota also necessary for keeping our immune system functioning by maintaining a low but constant expression MHC II on APC's resident microbiota cause stimulation of cross-protective antibodies, due to the PAMP's that microbiota share with other pathogens

Effector mechanisms in IgE mediated allergic reactions

common allergens that cause IgE mediated reactions include drugs, venoms, dander, food, pollens, etc responses vary based on whether allergen enters mucosa or respiratory tract

Location of Complement System Action

complement only functions inside the body (blood, interstitial space), not on the surface ∙inactivated in tears, saliva, mucins ∙should make sense: a consequence of complement activation is tissue damage, which we do not want happening to barrier epithelium ∙also explains why primary antibody that protects mucosal surfaces is IgA, a noninflammatory Ig ∙IgE is another antibody that is unable to activate complement system Although IgM and IgG are found circulating in blood, they are unable to activate complement in blood ∙not until antibody binds pathogen and undergoes conformational change (planar → crablike) that C1 binding site is exposed on its constant Ch2 domain of its Fc region The number of antibodies required for activation DIFFERS ∙only one needed for IgM activation ∙IgG needs 2, although usually several IgGs have to bind before two are lose enough to be bridged by C1 MBL uses a molecule containing MASP-1 and MASP2, similar in structure to C1 ∙both are made up of 3 structurally and functionally similar parts ∙besides this difference, MBL pathway follows same sequence of steps that lead to C3 convertase and C3b opsinin

Complement system

complement system uses three pathways to augment the immune system by facilitating pathogen clearance by antibodies and phagocytic cells

Dendritic Cells

created mostly in common myeloid lineage, although small percentage can be created following common lymphoid lineage immature dendritic cells function as phagocytic cells when they first enter tissue, however, they mature after encountering pathogens and function in presenting antigens to T lymphocytes → important APCs Note: mature dendritic cells are only APCs that can activate naïve T lymphocytes dendritic cell is most important link b/w innate and adaptive immunity b/c they are most potent of APCs

Testing for transplant sensitivity

crossmatch - involves taking patient serum and incubating it with cells of donor organ to see if there is reactivity can use this to see if the patient responds with antibodies, etc, against foreign HLA

Adjuvants

defined as any substance that enhance immunogenicity →necessary components of soluble subunit/toxoid vaccines (i.e. diphtheria, tetanus toxoid) →usually derived from microorganisms can make antigen more immunogenic through a number of strategies →include, but are not limited to, converting soluble antigen into particulate material, slowing release of antigen inside body, enhancing uptake of antigen by macrophages →microorganism component of many adjuvants works by ligating pathogen recognition receptors, which stimulate innate immune response Freund's complete adjuvant is most potent of adjuvants →contains mycobacterium in an oil-in-water emulsion (micelles) →no longer used in lab b/c of its potency Adjuvant we use in human vaccine is aluminum hydroxide →binds to vaccine antigen, creating particulate antigen complex that enhances uptake by macrophages →also creates depot effect by concentrating antigen at site of injection and delaying release into body

Linked recognition

dendritic cell uptakes pathogens non-specifically and presents them on MHC II to activate the naïve T-cells all that is required for the B-cell to present the same peptide on its MHC II means that the mechanism of how the B-cells require the peptide is irrelevant → eptiope spreading ∙as long as B-cell can bind to any epitope on that pathogen, it can phagocytize it and giest it to present on MHC II

Antibody Pool

determines which epitopes are selected by antibodies present in our body Ab pool is generated prior to antigen exposure only upon exposure to an antigen that a B lymphocyte becomes activated, leading to clonal expansion

B-Cell Development

develop in bone marrow, migrate to peripheral lymphoid organs where they are activated by antigens, then proliferating and carrying out their immune roles as B-cell matures, combines J, D and V segments from one of its chromosomes to try to develop a functional receptor ∙if it develops a functional receptor, it is tested against host stromal cells in bone marrow ∙if it does not react it is allowed to leave bone marrow ∙if it does react strongly, it is given a chance to reorganize its receptors in a process called receptor editing →in receptor editing, it ditches first receptor set it had developed and tries to create a new receptor using segments of second set of V, D and J segments on second chromosome ∙if lymphocyte again reacts strongly, it is signaled to undergo apoptosis B-cell might not be activated by host cells but might be activated by soluble host molecules ∙in this case, tolerizing reaction occurs and B-cell is released from bone marrow but is functionally useless, so it does nothing in body, eventually dying w/o fulfilling role If B-cell has low affinity for host cells, bone marrow decides that even though it has some reactivity towards self-cells, it could prove to be useful in immune response and releases it to periphery Once B-cells leave bone marrow, they express both IgM and IgD receptor types on cell surfaces ∙if lymphocyte finds antigen and binds with it, it down regulates IgD receptors and expresses only IgM receptors ∙B-cell then interacts with T-cell that releases cytokines to let B-cell know whether it needs to differentiate into a memory B-cell or Plasma, antibody producing cell

Transplant Directionality

donor can also reject recipient tissue in what is called Graft vs. Host Disease (GVHD) →mature immune cells could be hanging out on tissue donation, can reject recipient either acutely or chronically overall, rejection is generally a solid organ transplant problem on side of recipient rejection is much less of an issue in bone marrow transplantation, and rejection here is basically limited to GVHD

Removal of Immune Complexes in Complement System

during immune response, pathogen antigens and their pathogen-specific IgG antibodies form complexes that bind the C3b RBCs have complement receptors (CR2) for C3b, which capture there large antibody:antigen complexes and remove them from circulation as they pass through spleen and liver resident macrophages in these areas destroy complexes BC surface and dispose them

Eosinophils are known as tissue damaging cells in Type I responses

eosinophils located under barrier epithelia contain receptors for IL-5, IgG and IgG antibodies (Fcγ and Fcα, respectively) release prostaglandins, secrete Th2 type cytokines and other types, and IDO (indoleamine 2,3-dioxygenase) which is toxic to Th1 T cells main association in IgE mediated responses is that they cause significant tissue damage because they release toxic mediators, such as free radicals and other harsh substances

Spleen

essentially functions in same manner as generalized lymph nodes, with some key differences →spleen filters blood, lymph nodes filter lymph →spleen is site of hematopoiesis, unlike lymph nodes →does not have lymphatic supply or drainage spleen consists of red pulp and white pulp, where the white pulp functions as secondary lymphoid tissue basic anatomy of spleen is still the same, with B cell and T cell regions a major function in both spleen and lymph nodes is to allow T cells to contact APCs from peripheral circulation

Self peptides

even when there is no pathogen present in our bodies, MHC class I molecule presents endogenous self antigen and MHC class II molecule presents exogenous self antigen

Comparison of antigen recognition b/w T-cells and B-cells

for the chemical nature of antigen, remember that we are restricting our discussion to protein antigens

Immunogenic Antigen

generally, an antigen needs to >100kD to stimulate an immune response Antigen receptors →B cell receptors (BCR): membrane bound antibodies on surface of the B-cell which are capable of recognizing tertiary structural components of native soluble antigen →T cell receptors (TCR): membrane bound disulfide linked heterodimers of highly variable chains which recognize linear peptide antigen in the context of MHC →BCRs can recognize foreign molecules in their native 3D confirmation, TCRs recognize linear peptides in context of MHC

Understanding MHC Differences in Transplantation

genes controlling acceptance/rejection are mapped into MHC Class I & II kidney expresses these genes, which body recognizes as yours b/c they have so many alleles, tissue selection in regard to a donor becomes difficult since donors will look foreign unless you can manage to match up some alleles to complicate further, a transplanted organ's MHC's can look like your own MHC carrying a pathogen ∙if this is what body sees, you will activate memory cells that previously responded to pathogens and they will respond to new organ ∙response is stronger than to a pathogen alone because of cross-reactivity and activation of multiple T-cells which now target the new tissue ∙known as direct cellular recognition

Hapten-Carrier Relationships

hapten = small molecule that must be attached to protein carrier in order to stimulate a response this backfires when penicillin binds to RBC ∙attacking hapten means carrier is attacked, resulted in hemolytic anemia

Sources for HPC

harvesting blood marrow stimulating stem cells with growth factor, causing them to leave bone marrow and to go peripheral blood and then isolating peripheral blood with apheresis can use umbilical cord blood

Arthus Reaction is a localized response

has same mechanism as serum sickness but it is a localized response that occurs on skin surface if an individual has high circulation IgG antibodies with antigen, they can deposit these complexes in skin mast cells have the ability to bind to IgG antibodies, although their affinity for them is very low compared to IgE receptor Similar to IgE, when two IgG molecules that are bound with antigen cross-link, degranulation occurs, causing local inflammatory response efflux of leukocytes enters area → visible swelling Bee stings can cause this reaction, as well as environmental antigens that are inhaled in lungs, hypersensitivity can occur caused by inhalation of bacterial spores, fungi, dried fecal proteins ∙includes Farmers lung from moldy hay, bagassosis from moldy sugar care, or bird fancier's lung from droppings, feathers and serum proteins

Atopic

hypoallergenic

Local Infection: Sounding the Alarm

if microorganism can adhere and invade, or if epithelia is breached due to outside trauma, first resident cells that will be activated are the macrophages and dendritic cells ∙their job is to phagocytize and interalize those cells to kill them and they are assisted by a variety of antimicrobial proteins and factors that are constantly in the tissue fluid Macrophage - phagocytizes the pathogen and releases chemokines and cytokines to recruit and activate cells from blood ∙process is known as acute inflammation ∙acute inflammation is a hallmark of innate immunity and works to deliver additional effector molecules and cells to infectin site, provides physical barrier to prevent spread of infection, and promotes repair of injured tissue Dendritic cells - phagocytize pathogens and migrates to draining lymph nodes where the dendritic cells present peptides from the digested pathogen with MHC II molecules

Laws of transplantation

if you take graft from a donor mouse that is genetically identical to recipient, graft will be accepted →if done with allogeneic mouse, garft will be rejected in about 2 weeks 1st set rejection ∙graft introduced → revascularizes in 3-7 days → immune cells infiltrate around days 7-10 → start to see necrosis, damaged blood vessels and blood clots around days 10-14 ∙predominant target for rejection is vascular endothelium 2nd set rejection ∙this time, rejection will be swifter and stronger ∙takes about 1 week, result of immunologoical memory ∙mouse or human can produce this kind of rejection if sensitized by pregnancy or blood transfusion as well

Trademarks of autoimmune disorders

immune response does not differ in modes of attack/activation ∙uses same cells and mechanisms to attack self as it does to attack pathogens onset of autoimmune disease is chronic and progressive because activation is cyclic ∙we cannot simply get rid of self antigen

How does mucosal immune system differentiate between innocuous non-self (i.e. food Ags, commensal bacteria, etc) and pathogens?

immunosuppressive cytokines, TGF-β and IL-10

Major Histocompatibility Complex (MHC)

in humans called HLA (Human Leukocyte Antigen) has two types: MHC I and II MHC is a cluster of genes that control tissue compatibility MHC proteins play major role in adaptive immune response and the rejection of tissue grafts

Acute response in allergic asthma leads to Th2 mediated chronic inflammation

in sensitized individuals, during acute phase, mast cells release inflammatory mediators, which cause increased mucus secretion and smooth muscle contraction eventually, cells from circulation are recruited chronic response is produced when Th2 cells support inflammatory response by continuing production of cytokines and eosinophil products ∙can cause irreversible damage to airways Desensitization: process where increasing amounts of antigen is introduced in skin ∙applied on skin b/c if given intravenously, patient can have much more severe consequences → circulatory collapse, death ∙after antigen is rapidly applied, patient waits 30 minutes such that if anaphylactic shock happens, doctor can administer Epi

Left-sided shift

increase in immature neutrophils that indicates an active infection and subsequent immune response cytokines and chemokines signal immune cells to respond → bone marrow receives signals → increased production of neutrophils → immature neutrophils released into circulation

How does autoimmunity occur?

initiating events are unknown, there are a few hypotheses one theory is the burden of infection theory

Innate vs. Adaptive Immunity

innate immunity involves cells of the myeloid series, and works in a rapid onset, nonspecific manner adaptive immunity functions in slow onset, highly specific manner, involving B and T cells dendritic cell links two types of immunity by instructing adaptive immune system's actions NK cells function primarily in innate immunity

Cytokine

instructional molecules of immune system direct cell actions (i.e. differentiation, production of transcription factors)

Peyer's Patches in mucosal immunity

just like lymphoid tissues in the neck (with paracortical areas and germinal centers) except that they don't have any afferent lymphatics ∙no immune cells brought to Peyer's patches in lymph, instead, they are brought in by blood vessels they can however, drain out of the Peyer's patch via efferent lymphatics bare areas of Peyer's patches are M (microfold) cells ∙highly phagocytic cells, will take up anything they can get a hold of in lumen and hand off any luminal Ag to dendritic cells that site directly beneath them ∙dendritic cells are able to extend an arm in between entereocytes and pluck Ags out from luminal surface After dendritic cells present naïve T-cells with Ag the T-cells can either: ∙deliver helper to B-cells in Peyer's patch OR ∙go to the mesenteric lymph nodes (largest group of lymph nodes in body) →from here, cells leave mesenteric lymph nodes via lymphatics and enter circulatory system, T-cells circulate in blood and either return to mucosal surface or seed other sites

Immunoglobulin structure

large, multimeric glycoproteins most operate as Y-shaped monomers composed of 2 identical "HC" heavy chains and 2 identical "LC" light chains (kappa or lamba) ∙60% of LCs are kappa, 40% are lambda ∙chains, especially the hinge region (center of Y), are linked by cysteine disulfide bonds ∙lower trunk contains Fc (F crystallized) fragments and 2 prongs contain Fab (F-antigen binding) fragments →fragments contain part of constant region and at the tip contain the variable region →variances are what give variable region its specificity

Peyer's patches

located along mucosal lining, specically in small intestine →about 75% of all immune cells are in mucosal surfaces overall organization is the same as other secondary lymphoid tissues →still have B cell regions, T cell zone occupies area b/w follicles →have no afferent lymphatics, similar to spleen, because antigens can enter directly from gut through specialized microfold (M) cells antigens enter the patches through M cells, immediately encounter dendritic cells →dendritic cells can thus act as APCs and allow action of T cells to take over Peyer's patches have different efferent lymphatics to drain pathogens

Solid organ transplant

long-term survival is important consideration downward trend in survival or organs over time, clear that in general grafts slowly die off over time this is a problem b/c there is a shortage of organs

Immunoglobulin fragments

lower trunk contains Fc (F crystallized) fragments and 2 prongs contain Fab (F-antigen binding) fragments →fragments contain part of constant region and at the tip contain the variable region

Lymph Nodes

lymph node receives its lymph supply from an afferent lymphatic vessel dendritic cells that carry pathogen enter through afferent lymphatic vessel and display antigens to activate T lymphocytes blood supply is essential, as naïve lymphocytes are delivered to secondary lymphoid tissues in blood to be activated 3 major components of lymph node: →outer cortex, which is composed of B cells organized as lymphoid follicles →para-cortical region, made up of T cells and dendritic cells →medulla, made up of plasma cells and mature B lymphocytes →when immune response is occurring, there are also areas of intense B cell proliferation in outer cortex called germinal centers (or secondary lymphoid follicles)

Lymph Flow in Body

lymph system runs parallel to our body's circulatory system, as lymph is technically just filtered tissue fluid circulatory system and lymph system connect in thoracic duct, which runs from base of neck along vertebrae to the lumbar region, and eventually empties into left subclavian vein In lymph system, primary and secondary lymphoid tissues have separate functions →lymphocytes created in bone marrow and thymus are released into blood and are carried in circulation to secondary lymphoid tissues →at these peripheral tissues, lymphocytes can be activated by foreign antigens which have been filtered from the blood →**Adaptive response takes place in secondary lymphoid tissues**

Cell Markers

lymphocytes are extremely difficult to differentiate using LM, so we can use CD markers to distinguish them based on proteins they display on surface and thus, the antibodies with which they interact →CD means cluster of differentiation major CD distinctions we need to know are CD4+ and CD8+, which are involved in cell-mediated immunity ∙CD4+ cells are helper T cells, which can differentiate into T helper (Th) 1, Th2, Th17 and T regulatory (Treg) cells with the helper of specific cytokines ∙CD8+ cells are cyototoxic T cells, which act on foreign cells by secreting perforin to cause cell death →require activation by MHC-1 molecules to kill target cell, similar to NK cells →infection can lead to downregulation of MHC-1 molecules on cell surfaces, and subsequently become invisible to CD8+ molecules, however, NK cells specifically pick up on this downregulation and can still kill infected cells naïve T cells are activated by dendritic cells and differentiate into either CD4+ or CD8+ cells Th1 cells are important in activating macrophages →Th2 activates B lymphocytes to make antibodies →Th17 initiates inflammation →Treg is regulatory T cell that polices other cells in negative fashion

Testing for specificity

lymphocytes are not tested against cells in the brain and gonads since there are barriers b/w immune system and brain/gonads →immune system cannot enter AIRE genes Only 1-2% of lymphocytes make it into circulation after screening →other cells undergo apoptosis and are phagocytized by macrophages in bone marrow/thymus

IgM

major Ig in primary immune response found as a monomer on B cell membrane serum IgM is a pentamer, has 10 binding sites (2 per monomer), so it is extremely useful in agglutination of antigens

Secretory Immunoglobulin A (SIgA)

major player in body's mucosal immune system principal form: dimer extra components: ∙J "joining" chain, which joins two IgA monomers together ∙secretory component (SC), an extra glycoprotein, which is product of glandular epithelial cells that transport SIgA to external surfaces

Mast cell is critical cell involved in Type I responses

mast cells found underneath barrier epithelia and around small blood vessels contain high affinity Fc receptors for IgE ∙typically, IgE found bound to mast cells when antigen crosslinks 2 IgE molecules on mast cell surface ,it causes degranulation and release of chemical mediators which generally act on vascular epithelium and causes smooth muscle contraction ∙immediate, occurs in 30 minutes mast cell has many effector functions in different areas of body when degranulation occurs ∙common molecules released by mast cells include Th2 type cytokines (IL-4), as well as other cytokines, proteases, mediators like histamine and heparin, chemokines and lipid mediators such as prostaglandins and platelet-activating factor

Polyclonal B and T cell activation

mitogens (Activate T cells regardless of specificity) and superantigens (proteins that bind to and activate all T cells in an individual that expresses a particular set of receptor on Vβ chain) non-specifically activate B and T cells that may have self-reactivity ∙these molecules are generally components of bacteria and viruses ∙included are lipo-polysaccharides from gram negative bacteria that activate B cells, specifically those that secrete IgM

Opsonins

molecules that bind antigens and make them a target of the immune system Two to know: C3b and IgG Pathogen membrane bound C3b binds to its receptor located on macrophages and other phagocytic cells ∙However, no phagoctosis of this complex will occur unless a C5a molecule also binds to its phagocyte surface receptor ∙only when both of these signals are present will macrophage internalize pathogen There exist IgG-Fc receptors on the surface of macrophages in order to recognize those IgG antibodies that have bound their specific pathogen ∙these IgG-Fc receptors must be cross-linked for activation, they require a minimum of two IgG molecules for activation Deposition of BOTH of these opsonins on the surface provides a synergistic effect and is the best way to promote phagocytosis

Features of mucosal immune system

more than 400m² of mucosal surface in the GI tract alone majority of B activated B cells (>80%) are located in the gut, associated with the mucus membranes ∙majority of these cells are committed to IgA synthesis →more IgA is synthesized per day than all other Ig isotypes combined lymphoid tissue is intimately associated with the epithelium there are specialized Ag uptake mechanisms Even in resting mucosa, in absence of pathogen, immune cells are in an activated state there are a lot of regulatory T-cells present in the environment →Immunosuppression - in addition to many regulatory T-cells, there are a lot of inhibitory dendritic cells and macrophages that exert suppression on T-cell in the mucosal environment, therefore, you have to do something special to activate in immune response at barrier epithelia

Blood transfusions

most common type of transplant cells are fairly short-lived, don't have to worry about long term survival need to match blood types

Pathogens too large for APCs to phagocytize

most multicellular parasites are too large to endocytose eosinophils will layer ∙eosinophil is a mediator-releasing cell, activated by IgE binding and cross-linking, causes eosinophil to degranulate and tip all anti-microbial factors into larvae in attempt to kill it ∙cell undergoes whole cell degranulation, meaning entire cell essentially explodes

Neutrophils

most potent phagocytic cell in body, generally first type of immune response cell that is recruited to an active site of infection circulate in blood during basal activity, are recruited to site of infection by chemokines are major component of pus in zits engulf pathogens through phagocytosis`

How do B-cells become activated to specific antigen?

need two signals (order is important): 1 - from antigen: cross-linking of B-cell receptors (BCR = membrane antibody) 2 - from helper T-cells: CD40:CD40L interaction mainly if only one of signals is received, then B-cell is not activated as a self-security measure occurring in secondary lymphoid tissues (lymph nodes, spleen)

Somatic Hypermutation in B-cells

newer B-cells undergo somatic hypermutation in germinal center after activation with helper T-cell ∙POINT MUTATIONS in variable regions of newer B-cells due to proliferation, random mutations will occur ∙mutations will cause B-cell to have lower affinity, higher affinity or no change to its interaction with the antigen ∙most of the time, result in lower affinity or nonfunctional proteins B-cells with lower affinity for antigen will not be able to deliver signal I because it will be unable to bind antigen with cross-linking ∙will die via apoptosis B-cells with higher affinity for antigen will have selective advantage when trying to become activated by means of signal 1 and 2 ∙natural selection process for previously selected B-cell

Acute rejection

occurs in days to weeks

Somatic Hypermutation

occurs in gemrinal centers of secondary lymphoid organs triggered by activation of a B cell, and represents a high rate of mutation that occurs in VDJ and VJ segments of heavy and light chains nucleotide sequences of slightly higher and lower affinity for Ag are generated through process →higher affinity Abs will be rewarded with increased proliferation b/c of more rapid binding/strong signalling

Inappropriate expression of MHC2

occurs when cells other than APC express MHC2 ∙every cell has ability to express MHC2 from its germ line, but will express MHCI in normal conditions Clinical: ∙Type 1 diabetes/Graves →pancreatic beta cells (diabetes) or thyroid acinar cells (Grave's) may somehow present self peptide with MHC2 →possible with no co-stimulation →→→if environment is sufficiently inflammatory, there may be sufficient cytokines and such to kick naïve T cells over to become effector T cells

Molecular mimicry (antigen cross-reactivity)

occurs when peptides from digestion of microbe or the like presented on APC are so closely related to self antigen that a reaction T cell cannot tell the difference, resulting in autoimmune attack of self cells Clinical: ∙Gullain-Barre →acute paralysis is outstanding symptom and results following infection from Camplyobacter jejuni →terminal oligosaccharide sequences of Schwann cell membrane and C. jejuni cell wall are nearly identical, resulting in joint recognition →leads to IgA, IgM and IgG deposition on Schwann cells → complement activation → T cell/macrophage infiltration → bad news ∙Rheumatic fever →occurs following streptococcal infection →anti-strep antibodies are depositied in heart and cause myocarditis

IgG

of Ig's present in serum, about 75% is IgG has longest serum half life, only Ig capable of crossing placenta to protect fetus 4 subclasses: IgG1, IgG2, IgG3, IgG4 ∙characterized by very minor differences in heavy chains ∙most differences unimportant ∙IgG1 has highest serum concentration, IgG4 has lowest ∙IgG3 has shortest half life ∙IgG1 and IgG3 are best at activating complement system IgG has half life of about 20 days

Signaling for Reinforcements

once dendritic cells and macrophages recognize that a pathogen is present, it releases pro-inflammatory cytokines, chemokines and increases expression of MHC Class I and II, up-regulates costimulatory molecules necessary to signal T-cells, and up-regulates adhesion molecules ∙this occurs as soon as infection begins, regardless of whether adaptive immune system will be needed Epithelium and pathogens themselves play a role in alerting immune system that there is an infection ∙epithelia are not passive players, instead produce variey of inflammatory/immunregulatory factors, chemotactic factors and stress proteins upon damage ∙all signal to immune system that there is problem and that epithelia are under stress/attack ∙pathogens produce similar factors upon destruction that alert immune system after they have entered body Chemokines can attract polymorphonuclear leukocytes (neutrophils), the dominant white blood cell circulating in the blood ∙neutrophils are no good in blood, so cytokines and chemokines from macrophage will recruit these cells to infected tissue Movement of dendritic cells from infected tissue to secondary lymphoid tissue signals beginning of adaptive immune response ∙in secondary lymphoid tissue T- and B-cells are activated for adaptive immunity ∙**without innate immunity, you cannot have adaptive immune response**

Other functions of the complement system

one of the receptors for complement, CR2, is part of the B-cell signaling mechanism ∙this surface B-cell receptor has no cytoplasmic tail, rendering it unable to transduce signals on its own ∙when B-cell meets antigen that is coated with compliment, B cell has an opportunity to be bridged via its complement receptor ∙this binding, along with binding of other molecules, delivers a powerful activation and maturation signal to B-cell in periphery T cells possess receptors that have the ability to recognize complement components ∙upon complement receptor activation, T cell is driven to become a regulatory T-cell

How do we ensure B-cell presents correct peptide on MHC II?

only a few variations of MHC molecules are shared b/w APC, so the range of peptides they can load are similar, therefore, likelihood that dendritic cell and B-cell will load the same peptide is high use it to our advantage ∙children has difficult time respond to TI antigens, but extracellular bacterial pathogens such as H. influenza are TI antigens ∙researches tried to make vaccine, but it only induced a TI response, creating only IgM and having poor effect in children →problem b/c pathogen required IgG antibodies to be opsonized to effectively eliminate infection

Epithelial Barriers & Immune Exclusion

pathogens mostly invade the body via barrier epithelia of either the skin or mucous membrane easier to pass through mucosae because it is much thinner (typically one cell thick), whereas the skin is thicker, dry and heavily keratinized ∙mucosae also make up a much larger surface area of our body than does our skin favored route of infection is through mucosae of respiratory tract (nose/mouth) ∙about 2/3 of infections Desquamation - acts to carry away any microorganisms that may have colonized, occurs along both skin and mucosa ∙both surfaces also release secretions with antimicrobial factors ∙when surface of mucosa is infected, the flow rate will increase along mucosal membranes and create a flushing action →do not consider sweat from skin as a flushing action both epitelia types also contain pattern recognition receptors (PRR) which are receptors that recognize conserved structural motifs on surface of pathogen ∙motifs are called pathogen associated molecular patterns (PAMP) or microbial associated molecule patterns (MAMPS) ∙PAMPs typically highly conserved structures since mutations can often lead to a critical loss of function and cell death

IgE mediated allergic reactions can have rapid onset but can also lead to chronic responses

peak expiratory flow rate (PEFR) is initially high IMMEDIATE PHASE: antigen is inhaled → PEFR drops rapidly ∙signifies bronchial constriction ∙we see restoration for quite some time before late phase occurs LATE PHASE: PEFR drops again ∙continued production of inflammatory mediators sustains late phase, which leads to continued vasodilation, produces edema

Macrophages

phagocytic molecule of myeloid lineage - tissue of variety of blood monocyte distributed through body in all tissues, very long-lived macrophages that reside in a specific tissue have differentiated names, while other macrophages wander b/w tissues macrophages can also double as antigen-presenting cells (APCs) engulf pathogen through phagocytosis

Mast Cells

play important role in inflammation full of granules containing pharmacologic reactive mediators that act on smooth muscle and endothelium activation by complement cascade allows mast cells to secrete thse granule cells increased vascular permeability from mast cells allows for up-regulation of cell adhesion molecules and for an increase in affinity of receptors to trap leukocytes, allowing them to then squeeze into tissues and follow chemokine pathway to infectious agents also involved in allergic reactions

Features of Innate Immunity

primitive system that has been conserved through evolution part of its effectiveness comes from its ability to respond rapidly to infection (minutes to hours), which it achieves through use of germ-line gene products that are inborn, not learned ∙genes can be transcribed and translated immediately and allow the innate immune system to recognize evolutionarily conserved motifs that are present on various types of pathogens and are not found on own cells Not specifically directed against an invading pathogen, but instead recognizes patterns with low specificity ∙low specificity gives the advantage of recognizing a broad coverage of antigens, but unfortunately allows for potential of collateral damage since the innate immune system CANNOT differentiate self and non-self Innate immunity retains no immunological memory, will always respond in the same way

Natural Killer Cells

principle immune response in absence of MHC molecules ∙seen in viral infection, malignancy can kill cells via antibody dependent cellular cytotoxicity (ADCC) antibodies attach to infected/target cell, then NK cell can recognize the Fc portion of the antibody cross-linking of multiple antibodies will activate NK cell and it will secrete factors that will cause apoptosis of target cell

Properties of T-cell antigens

professional APCs are group of immune cells that are programmed to take up antigen, process tem, and present antigen derived peptides to CD4+ T-cells in the context of MHC class II →includes B-cells, macrophages, and dendritic cells →B cells are converd with antibody which allows for receptor mediated endocyotsis of native and soluble antigen →Dendritic cells and macrophages are phagocytes that phagocytose bacteria and dead cells →the processing of exogenous antigen takes place within lysosomes which fuse with phagosomes within the APC Ability to be degraded/metabolized →APC must be able to breakdown and degraded antigen in lysosomes in order for it to be presented on MHC Structural Organization →antigens have some degree of organization or rigidity to be recognized and taken up by APC Solubility/Accessibility →macrophages and dendritic cells engulf primarily insoluble or particulate antigens in colloidal suspension (bacterial cells, viral/fungal particles →→→these antigens are more immunogenic than soluble antigens (not to say that soluble antigens cannot be immunogenic, BCRs are primarily responsible for uptake of soluble antigen)

Chronic Inflammation

provides positive feedback due to inability to clear self antigen ever also serves to exacerbate immune response for the rest of individual's life B cell in circulation bind to peptide from dying cell → B cell is activated by T cell → new plasma cells release lots of auto-antibody → inflammatory response at specific tissue → cycle continues

Sex Bias in Autoimmunity

remember that SLU, MS, Grave's Grave's and Hashimoto's all occur more frequently in women due to genetics associated with MHC2 genes which determine the types of peptides that can be presented Development of SLE due to DR3 allele leaves women 10-20x more at risk B27 is also a key gene ∙presence of this gene leaves you as 88x greater risk of Ankylosing Spondylitis

Release of sequestered antigens (from immunoprivileged areas)

remember that these areas are the testis, retina, brain, uterus

Serum sickness is a systemic response

serum sickness occurs when a patient receives foreign serum, usually given to provide passive immunity ∙systemic response in the past, horse serum that contained antibodies against pneumococcus was given to individuals that had severe cases of the disease ∙the serum was recognize by patient's immune system as foreign and therefore antibodies were produced against it during the initial stages of this scenario, antibody production is slow so there is a case of antigen excess immune complexes are readily deposited into capillary beds found in kidneys, lungs, joints, choroid plexus, etc. ∙cause inflammation of these sites, causing fevers and symptoms of vasculities, nephritis and arthritis eventually, equivalence b/w antigen and antibody is reached ∙antigen can now be cleared from system efficiently

Plasma cell formation

short lived in medullary cords long lived migrate to bone marrow

TH-Cell (Helper/CD4+) Action

should focus on learning cytokines released by Th-cell that have come up multiple times know 5 on slide 22, as wellas IL-6, TGF-alpha, TGF-beta

DH response

similar mechanism to contact hypersensitivity antigen is injected into skin, processed by local APCs ∙Th1 cell that has already been primed against this antigen recognizes it, releases cytokines that activate local endothelial cells ∙macrophages are recruited to site and cause visible lesion that differs from wheal and flare reaction in that it is firmer common example: mycobacterial proteins that are present in TB test ∙individual who have been exposed to TB will have inflammatory reaction occur at site of injection

Primary lymphoid organs

sites of lymphocyte development two major primary lymphoid tissues are bone marrow and thymus (located in superior medastinum above heart)

Chronic rejection

slow rejection takes months to years

Conjugate Vaccine

solution to problem with children and TI immunity physically linked polysaccharide of H. influenza to protein (tetanus toxoid) ∙MUST BE PREVIOUSLY IMMUNIZED in this case, BCR still recognized the polysaccharide, but it would endocytose the entire molecule, including tetanus toxoid protein peptide from toxoid are then presented on MHC II molecule to initiate a TD response, which included secretion of cytokines from helper T-cell that allowed B-cell to undergo class switching Now in B-cell, specific for H. influenza, are differentiated into plasma cells, capable of secreting IgG antibodies (due to TD response induced by toxoid) ∙therefore, able to effectively opsonize and eliminate future infection

Contact hypersensitivity

some types of contact hypersensitivity agents are haptens that bind to self-proteins, such as irritants in poison ivy ∙small metal ions can also induce response contact-sensitizing agent enters the skin and binds to self-proteins ∙agent acts like hapten-like molecule ∙agent bound to self-protein is taken up by skin Langerhans cell (type of dendritic cell), present haptenated self-peptides to Th1 cells, which have already been primed ∙Th1 cells secrete IFN-γ and other cytokines, which act on epithelium, which releases pro-inflammatory cytokines ∙macrophages become activated, secrete mediators that lead to inflammation →can be damaging → ulceration of skin

Complementarity b/w BCR and TCR

strength of binding for BCR and TCR is function of complementarity the binding we observe in BCR-epitope and TCR-epitope-MHC cinteractions in non-covalents, and reversible

Termination of the complement system

termination of complement cascade involves assembling a Membrane Attack Complex (MAC) ∙begins with C5 voertase that generates C5b ∙C5b binds C6 and C7, followed by C8, which is inserted into the membrane which is inserted into the membrane instead of simply binding the surface ∙finally, up to 16 molecules of C9 polymerize in a circle to form a pore

Epitopes: antigenic determinants

the individual parts of antigen that immune system recognizes single antigen may have many epitopes can be either conformational or linear secondary classification system of dominant, subdominant, and cryptic

Action of Th1-cell

they are CD4+ T-cells that help macrophages combat infections and help B-cells produce IgG1 and IgG3 when a macrophage is having trouble using its phagosome, containing the pathogen, with its lysosome, it will express some pathogen peptide on its surface to signal to Th1-cells it is having difficulty Th1-cell will release IFN-gamma, which enables macrophages to fuse its phagosome to its lysosome and destroy engulfed pathogen and fully combat the infection

Hematopoietic Cell Transplant (HPC)

transplant of blood forming cells transplant can help us if we are facing something like fatal blood disease

Preventing an Immune Response in Transplantation

try to match MHC (same alleles) and ABO groups in blood transfusion, helpful to separate blood components so you're not exposed to WBC's that could express MHC in platelet transfusion, you will be exposed to platelets carrying foreign MHC molecules ∙if you are getting a lot of these you need to be matched up with someone that has similar MHC alleles to prevent sensitization and overreaction in HPC transplant, precise HLA matching is important to prevent immune response since you need to generate a new immune system for person in question ∙matching probability varies depending on commonness of alleles a person has ∙even mismatching one allele can reduce survivability In solid organ transplant you rely more on immune suppression than HLA matching ∙exact matching makes organ finding a tougher process, so more emphasis is placed on reducing sensitivity and immune suppression ∙limitations to immune suppression: →toxic drugs, increased susceptibility to infection, failure to reconstitute an immune system after HPC transplant, cost, patient compliance

Type IV reactions are cell mediated

two different forms: ∙contact hypersensitivity ∙delayed hypersensitivity (DH) in both cases, same sort of response is initiated, response takes time to develop (36-48hrs) difference is the route of entry ∙Contact hypersensitivity - skin absorbs antigen ∙DH - antigen is injected into skin

Light Chain Gene Rearrangement

variation derives from V and J exons ∙about 40 possible V exons, 5 J exons once differentiated to B lymphocyte from stem cell/epithelial cell's DNA, DNA is permanently rearranged one V segment is spliced next to ONE J segment

Gene Rearrangement (heavy/light chains)

variations seen in variable region are result of randomized "mix and match" of a few exons from a couple distinct categories both heavy and light chains undergo gene rearrangement and both contribute to variable region variation, which is why there are so many gene combinations and why Ab pool is so diverse variable region of light chains derive from V and J exons variable region of heavy chains derive from V, J and D exons **DO NOT CONFUSE J EXON WITH J CHAIN INVOLVED IN IgM PENTAMERIZATION OR IgA DIMERIZATION**

Interferons and Anti-Viral Immunity

when cell is infected by virus, it detected double stranded RNA using cytosolic PRRs and produces cytokines called interferon-alpha and interferon-beta interferons render uninfected cells resistant to viral infection and upregulate MHC Class I to display antigen to T-cells interferons also activates NK cells to kill virus-infected cells Viruses have tried to become sneaky by inhibiting an infected cell's ability to display MHC I ∙every cell in that body that has a nucleus expresses MHC Class I ∙only cells in immune system produce MHC Class II ∙keep in mind that MHC Class I molecules will monitor and display cytosolic peptides whereas MHC Class II will display molecules from phagocytosis products or pathogens that originated from outside the cells, this allows immune system to determine if infection is from insode or from outside cell ∙by blocking infected cell's ability to display MHC I molecules, virus can mask its presence in cell ∙however, our immune system is smarter than virus and uses NK cells to kill infected cells that are not displaying MHC I molecules

Routes of allergen entry

when high does of allergen is introduced intravenously, there is systemic release of connective tissue mast cells, which can cause anaphylactic shock a low dose entry of an allergen via the skin will result in a local release of histamine by connective tissue mast cells that cause a wheal and flare reaction ∙wheal and flare reaction = edema and redness caused by vascular permeability and vasodilation of blood vessels inhalation of allergen will result in increased mucous production and bronchial contraction mediated by mucosal mast cells Ingestion of allergen will result in mucosal mast cells eliciting intestinal smooth muscle contraction, which induces vomiting or diarrhea

Accelerated Acute Rejection

where an individual is already sensitized and has memory cells from previous rxn become activated upon contact with new graft (takes place over a week) and you see lymphocyte infiltration and complement C4d deposition in tissues has potential to destroy graft happens within first week

Secondary lymphoid organs

where immune cells meet to produce an immune response organs are lymph nodes, spleen, mucosa, tonsils, Peyer's patches, appendix