Block 2 - Immune System

Complement Proteins

20 proteins made in liver complement cascade produces C5b Cb5 Cb3

**What lymphocyte type is responsible for humoral immunity? Why?

B lymphocytes they release antibodies into the extracellular fluid and the blood. "B, you humor me"

**Where do B lymphocytes developed?

Developed - bone marrow

Natural Killer Cells

Natural Killer Cells are a subtype of bone marrow derived (B) type lymphocytes that takes part in the innate immune system. Function - eliminating cancer cells and cells infected with virus *** They do not produce antibodies or recognize specific antigens Instead, they display two types of transmembrane receptors: 1. killer-activating receptors 2. killer-inhibitory receptors ****NK cells - have Fc receptors that attach to IgG that allows them to recognize the cell tagged by antibodies

**What enzyme does Cytotoxic T cell secrete after recognizing the MHC I on a virus?

Perforin - produce channels in the infected cell's membrane Granzymes - enter through these channels and induce apoptosis in the infected cell.

Not Inflammation

Recall that mast cells secrete neutrophil and eosinophil chemotactic cytokines factors These cytokines cause the two white cell types to leave the blood vessel in the area of inflammation, and migrate towards the area of infection. Once there, the neutrophils can help destroy the invaders and the eosinophils then act to keep the responses of the defensive cells under control. In particular, the eosinophils turn off the attack, once the invaders have been routed.

1. Selectins

Selectins interact weakly with cell surface molecules on white blood cells (neutrophils, monocytes and eosinophils) causing them to roll along the surface of the vessel and slow down.

Mechanisms of Antibody Action: without involving other systems:

Shown here are three ways in which antibodies can defend the body following antigen binding without involving other systems: 1.Neutralization: The antibody can simply attach to components of a virus that the viral particle needs to infect cells, rendering it harmless. 2.Agglutination: It cross-links bacteria so that they can no longer replicate. 3.Precipitation: It can attach to toxins, making them insoluble and or inactive. This is how anti-venoms work.

Adaptive Immune Response Time Course

Slow antigen recognition lymphocyte activation antigen elimination contraction (hemostasis) memory

**The binding of the CD 40 to T helper cell receptors causes what?

T helper cell to release a host of interleukins stimulate the now activated B cell to proliferate. ***This is called clonal expansion The progeny of this activated B cell become two types of cells: plasma cells and memory B lymphocytes Plasma cell - producing antibodies that have the same antigen specificity as the one that was activated at the beginning of this process Memory B lymphocytes - have long lives and are there to fight future infections by the same micro-organism.

Cb3

The C3b component can also attach to a bacterium, opsonizing it to induce neutrophil and macrophage phagocytosis. **Cb3 tags the bacterium for phagocytosis

**how does B cells gets triggered?

When two of the IgD antibodies on the surface of a B cell are able to recognize an antigen, it triggers the cell to place an epitope of that antigen on its major histocompatibility complex II (MHC II).

Secondary Lymphoid Follicle

Whenever a B lymphocyte is activated following the steps just covered, it undergoes clonal expansion. It produces a histological structure called a secondary lymphoid follicle ****follicle contains a lighter Germinal Center where the B and T cells are multiplying rapidly ****darker surrounding Mantle, where they are undergoing differentiation before moving off to do battle with the offending organism.

Chemokines

are cytokines that attract other immune cells.

**The binding of the CD 80 to CD 28 receptors causes what?

causes the T helper cell to release chemokines that attract more T heper cells to the area

response to phagocytosing micro-organisms

macrophage becomes activated and releases TNFα and Il-1. This induces endothelial cells to express selectins and ICAMs on their plasma membranes.

**When a macrophage is activated with a MHCII from a helper T cell that is the same as the MHC II on it's receptor, what does it secrete?

macrophage will secrete IFN gamma and TNFalpha

**do people with HIV die due to HIV? explain

no, the HIV attacks the helper T cell through the CD4 and causes decreased immunity people die because of opportunistic infections

MHC II

only found on lymphocytes and antigen presenting cells - like macrophages **recognizes NON-SELF These cells place short exogenous peptide sequences from invading organisms as the epitope on their MHC II. **Know process When MHC II is made in the rER, a molecule called CLIP is loaded into its groove. In this case, the epitopes are produced by lysosomal degradation of exogenous proteins (e.g., bacterial coat proteins) in phagosomes/late endosomes. They are then transferred to the vesicles containing the MHC II molecules, and replace the CLIP, before the MHC II is placed on the cell surface, tagging it as "non-self".

killer-activating receptors

recognize a number of markers on nucleated cells for the NK cell to be activated These NK cells only attack cells if the killer-activating receptors are engaged

**Where are lymphocytes stem cells located?

stem cells producing both populations (B or T cells) are located in the bone marrow

lighter Germinal Center

where the B and T cells are multiplying rapidly

Mechanisms of Antibody Action: antibodies interact with other systems

***uses the Fc receptor 1.Complement fixation: the Fc region activates complement proteins to form membrane attack complexes. 2.Opsonization: Fc receptors on antibodies trigger phagocytosis by macrophages and neutrophils. 3.Natural Killer activation: Fc receptor binding trigger these cells to kill identified invader.

**What does Activation by interleukin 2 causes?

Activation by interleukin 2 causes the T helper cell to multiply and seek out other macrophages with the epitope attached to MHC II. When the T helper cell attaches to this epitope, the macrophage pumps out interferon gamma. This interferon causes the macrophage to auto-stimulate using TNF alpha. As a result of this signal, its lysosomes fuse with the phagosomes, destroying the bacteria

Mast Cells exocytosis

Activation of multiple IgE receptors stimulates a pathway in which adenylate cyclase makes cyclic adenosine monophosphate (cAMP) from adenosine triphosphate (ATP). The cAMP stimulates an influx of calcium ions (Ca+2) to produce exocytosis release histamine, heparin, and a variety of proteoglycans that can act as chemotactic factors for other immune system cells.

3. At this point the white blood cell moves between the endothelial cells

At this point the white blood cell moves between the endothelial cells to exit the vessel and enter the tissue. This process of passing through the endothelium is referred to as diapedesis or extravasation.

darker surrounding Mantle

B and T cells undergoing differentiation before moving off to do battle with the offending organism.

Cytokines

Cytokines are signaling molecules released by the cells in the innate and adaptive immune system. They modulate the activity of immune system cells, and recruit more of these cells.

**Where do T lymphocytes developed?

Developed - thymus Immature cells destined to be T cells travel via the circulatory system from the bone marrow to the thymus, where they mature.

****NOT PROFESSIONAL ANTIGEN PRESENTING CELL:

Follicular Dendritic Cells are, as the name implies, found in lymphoid follicles. They do not phagocytose micro-organisms and they do not use the MHC II presentation system, so they are not considered "professional antigen presenting cells". They are, however, able to present exogenous antigens on their membrane surface, to activate B and T cells. Pretty good for an amateur.

**The Cytotoxic T cell recognize what?

MHC I stabilized by CD4 and B7 (on antigen presenting cell)

**What is the process happening here?

Margination & Diapedesis Examples of marginating white blood cells in a venule indicated by blue arrows. An extravasating cell is indicated by the green arrow.

In mammals such as ourselves, there are two additional defensive systems.

1.Innate immune system 2.Adaptive immune system

What produces antibodies? Describe the antibodies.

Antibodies are produced by B lymphocytes; have two forms: ** membrane bound form ** form that are secreted in the ECM fluid Antibodies are made up of identical dimers attached to each other by disulfide bonds. Each monomer consist of a heavy chain and a light chain. The light chain consist of a constant region and a variable region. ****It is the light chain variable region that attaches to the antigen to give this molecule its specificity. The heavy chain allows the antibody to interact with other immune system components.

Hypermutation (clonal selection)

B lymphocytes actually mutate the DNA coding for the amino end of the light chain where the recognition of the antigen occurs, a process called hypermutation. As a result, some B cells will produce even better, more specific, antibodies. The dendritic cells then help these more effective clones to continue to multiply and they suppress the less effective ones. **occurs between germinal center and the mantle border

Macrophage Activation by T Helper Cells

Bacteria that have been phagocytosed by macrophages are initially only slowly degraded, and can even replicate in the phagosome. However, once the macrophage places part of the bacterial protein as an epitope on its MHC II molecules, a T helper cell with the appropriate receptor can recognize the epitope as foreign. In this case, the interactoin is stabilized by CD 4. The T helper cell then expresses interleukin 2 receptors and secretes this interleukin to autostimulate itself.

Bradykinin

Bradykinin also indirectly activates pain receptors leading to pain and tenderness in the affected region (dolor). "Brady is hurting me"

Cb5

C5b then works together with other members of the complement family to form a ***Membrane Attack Complex. The membrane attack complex is able to poke holes in the membranes of single-cell organisms and parasites, killing them

**What receptors on the B cell

CD 40 and CD 28

**What receptors on the B cell binds with the receptors on the T helper cell?

CD 40 binds T helper receptors CD 28 binds with CD 80 (on T helper cells)

What receptors are on the T helper cell

CD 80 and T helper cell receptors

**What is present on the Helper T cell that recognizes MHC II?

CD4

Mast Cells - phospholipase pathway

Crosslinking of the receptor activates a phospholipase pathway which cause membrane bound phospholipids to be converted into arachidonic acid. The arachidonic acid is then converted into leukotrienes and prostaglandins. These also help mediate inflammation and allergic reactions.

***Karposi's sarcoma

HIV immunocompromised individual can have a normally rare, Karposi's sarcoma, which often occurs in the oral cavity.

Adaptive immune system

Highly specific Slow, because it requires steps Has a Memory Improves with each exposure encounter with a particular microbe leads to a much swifter and more effective response to that same microbe in the future 1.Phagocytic cells. Macrophages and Neutrophils work much more effectively when antibodies tag the enemy. 2.Lymphocytes. These antibody producing cells are responsible for the specificity and the memory characteristics of the adaptive immune system. 3.Cytokines. These signaling molecules allow the cells to coordinate their attack. *NK cells - have Fc receptors that attach to IgG that allows them to recognize the cell tagged by antibodies

***********Human Immunodeficiency Virus: HIV

Human Immunodeficiency Virus (HIV) attacks T helper cells, by attaching to the CD4 molecule on their membranes. It then inserts itself into the T helper cells and takes them over for production of more viral proteins. As the disease progresses, there are fewer and fewer T helper cells available to program cytotoxic T cells to attack infected T helper cells, until eventually this virus can disable the entire immune system by attacking the key T helper element. The patients don't die from the HIV itself, instead they succumb to a variety of opportunistic micro-organisms and/or tumors that can now grow in the immunocompromised individual. A common one is the, normally rare, Karposi's sarcoma, which often occurs in the oral cavity.

The two systems interact and support one another.

If a single bacteria divides every 30 minutes (slow for a bacteria), after two days you will have about 8 x 1028 bacteria. The innate system slows this reproductive rate down, so that the adaptive immune system will have a chance to bring its weapons on board to finish the job.

**What is the key player of adaptive immune response?

LYMPHOCYTES B cells and T cells allow our bodies to recognize foreign invaders, and they both directly attack the invading organisms and marshal the other cells of defense to attack them.

**A T helper cell recognizes what?

MHC II CD4 and MHC II CD 28 and B7 (on antigen presenting cell)

Healing

Macrophages also play a role in healing. neutrophils undergo apoptosis (programed cell death). The dying neutrophils attract macrophages which then ingest them. These macrophages are then reprogrammed into anti-inflammatory macrophages. They stop secreting TNFα, IL-1 and IL-8. Instead, they secrete IL-10. This interleukin change causes the endothelium to return to normal, so that white blood cells are no longer recruited. In addition, the anti-inflammatory phenotype of the macrophage secretes transforming growth factor beta (TGFβ) and fibroblast growth factor (FGF). These cytokines stimulate fibroblasts to repair the damage done by the infection and the actions taken to fight the infection.

Macrophages in Response to Infection

Macrophages are multifunctional cells. They: •Phagocytose invading organisms •Phagocytose senescent cells •Clean up after neutrophils •Present antigens to lymphocytes •Release cytokines to facilitate immune responses and inflammation The cytokines released by macrophages include: Tumor Necrosis Factor alpha (TNFα) and Interleukin 1 (IL-1). These cause the endothelial cells to express: Selectins and Intracellular Adhesion Molecules (ICAMs). These are crucial to recruiting leukocytes to the area of infection.

Inflammation

Mast cells and basophils have a central role in the process of inflammation ****first step in the inflammatory process occurs when a lymphocyte contacts the antigen it is specific for. The lymphocyte then converts into a plasma cell and makes antibodies of the IgE type. These are the ones that bind to the mast cell's IgE receptors.

Mast Cells Malfunctions

Mast cells are an important part of our immune defense system, but when they malfunction they can produce allergies or asthma

**NK cells have what receptor that attached to what?

NK cells have Fc receptors that attaches to IgG allows them to recognize cells tagged by antibodies. Thus, they also contribute to the adaptive immune system.

Innate immune system

Non-specific Fast, because its already there No Memory Can't be improved 1.Complement proteins. These are a system of blood-borne macromolecules. 2.Antimicrobial compounds. These are synthesized and released by epithelia; eg., Lysozyme & Defensin. 3.Phagocytic cells. Macrophages and Neutrophils can attack so micro-organisms without the help of antibodies. 4.Natural Killer Cells. These can attack virally infected cells, parasites, bacteria and tumor cells.

****PROFESSIONAL ANTIGEN PRESENTING CELL:

PROFESSIONAL ANTIGEN PRESENTING CELL: Macrophages are an antigen presenting cell that you have already met. They present antigenic epitopes on MHC II Dendritic Cells are a subset of fixed macrophages that spend a long time in certain tissues, acting a sentinels. They also can digest invaders and present epitopes on their MHC II. Epithelial Reticular Cells are found in thymus, and will be discussed in the lymphoid lecture. They use MHC to select non-self T lymphocytes.

Inflammation Responses

Redness (Ruber) Swelling (Tumor) Heat (Calor) Pain (Dolor) responses protect the body and bring more of the immune system transient cells into the connective tissue at the site of infection. They are intended to be local When they become too widespread in the body, the result is anaphylaxis, which can soon lead to death as serum drains into the extracellular space and the blood pressure collapses.

**IgA

Structure: Dimer with J chain and secretory component Antibody percentage: 10-15% Sites: Secretions (saliva, milk, tears..) Function: protects mucous

**IgD

Structure: Monomer Antibody percentage: 0.001% Sites: surfaces of B lymphocytes Function: Antigen receptors triggering initial C cell activation

**IgE

Structure: Monomer Antibody percentage: 0.002% Sites: bound to surface of mast cell and basophil Function: destroy parasitic worms and participates in allergies

**IgG

Structure: Monomer Antibody percentage: 75-85% **most abundant Sites: fetal circulation in pregnant women Function: ativated phagocytosis, neutralizes antigens *NK cell

**IgM

Structure: Pentamer Antibody percentage: 5-10% Sites: B lymphocyte surfaces (as a monomer) Function: first antibody produced in initial immune response; activated complement "Mom is always first"

**What lymphocyte type is responsible for cell mediated immunity? Why?

T lymphocytes protect us from viral invasion by attacking cells that contain viruses. They may also protect us from cancerous cells.

What produces T cell receptors?

T lymphocytes produce very similar (to antibodies) molecules called T cell receptors. **always bound to the cell's membrane

Somatic Recombination

The body produces an enormous variety of antibodies, that are able to recognize an enormous number of different antigens However, each individual lymphocyte only makes one specific antibody that recognizes one specific antigen ***These are produced by rearrangements of the DNA called somatic recombination one gene from the library is selected for each piece randomly. This results in an enormous number of different combinations. Once the somatic recombination is completed, the lymphocyte is now programed to make just one antigen-specific antibody.

2. The rolling cells eventually move slow enough that ICAMs can interact with integrins

The rolling cells eventually move slow enough that ICAMs can interact with integrins in the white cell membrane. This forms a strong bond that then holds the cells in place. Histologically this is referred to as margination.

4. The white blood cells then move to the infection site using chemotaxis

The white blood cells then move to the infection site using chemotaxis, in response to the concentration gradient of IL-8 secreted by the macrophage.

**Describe how Lymphocytes are activated?

Two step activation Lymphocytes often require two stimulus events in series before the cells multiply and begin doing there jobs. This two step process is directed at making sure that the immune system only attacks invaders. ex: 1. helper T cell activated 2. helper T cell activates Cytotoxic T cell

**How are different classes of immunoglobins produces?

Variations in the constant domain coding for the heavy chain at the carboxyl end (sometimes referred to as the Fc part of the molecule) create the different classes of immunoglobulin

***KNOW THIS PROCESS T Cell Dependent Plasma Cell Production

When two of the IgD antibodies on the surface of a B cell are able to recognize an antigen, it triggers the cell to place an epitope of that antigen on its major histocompatibility complex II (MHC II). These B cells express - CD 40 and CD 28. B cell encounters T helper cell. T helper cell recognize the MHC II epitope complex -- These T helper cells express - CD80 and T cell receptors T helper cell receptor binds the CD 40 CD 80 (of T helper cell) binds CD 28. The binding of the CD 40 causes the T helper cell to release a host of interleukins. The binding of the CD 80 causes the T helper cell to release chemokines that attract more T heper cells to the area. **Note the fact that the T helper cell is critical to the B cell activation. If all the other membrane components are not there, the B cell is not activated. In this way, T helper cells are crucial to making sure that B cells bearing antibodies to self, are not activated, preventing autoimmune responses. The interleukins stimulate the now activated B cell to proliferate. ***This is called clonal expansion. The progeny of this activated B cell become two types of cells: plasma cells and memory B lymphocytes. The plasma cells begin producing antibodies that have the same antigen specificity as the one that was activated at the beginning of this process. The memory cells have long lives and are there to fight future infections by the same micro-organism.

Cytotoxic T cell Activation

You need a antigen presenting cell that has processed the antigen and placed it as an epitope on both it's MHC I and MHC II. The Cytotoxic T cell recognize the MHC I epitope - relationship is stabilized through interactions of CD28 and B7 A T helper cell recognizes the MHC II epitope - involves CD4 bind with MHC II - stabilized by CD28 and B7 ***T helper cell secrete interleukins to activate the Cytotoxic T cell

class switching

ability of a B cell (lymphocyte) change the type of antibody they produce, but not the antigen it recognizes example: B cells usually have IgD attached to their membranes, but once they turn into plasma cells, they mainly produce IgG.

Histamine and leukotrienes

activated mast cell causes local vascular dilation increased blood flow, which is in turn responsible for the redness (rubor) and heat (calor).

Histamine, leukotrienes and bradykinin

affect the tight junctions between capillary endothelial cells, causing the endothelial lining to become leaky and allowing plasma proteins to leak out of the vessels This upsets the normal movement of water, increasing the amount of water pulled into the extracellular space. The result is swelling (tumor).

Colony stimulating factors

are cytokines released by immune system cells that stimulate hemotpoiesis- the production of immune system cells.

Interleukins

are cytokines released by lymphocytes. Some induce production of more lymphocytes.

Interferons

are cytokines that display antiviral effects.

First line of defense:

barriers epithelia and underlying connective tissue represent a physical barrier to invading organisms

**When cytotoxic T cell is activated by interleukins from the T helper cell, what happens?

cytotoxic T cell then undergoes clonal expansion, and searches for cells that present the same epitope on their surfaces Cells that have been invaded by viruses and are producing viral particles, have viral epitopes presented by MHC I The T cell attaches to this with its surface receptors and secretes it products: Perforins and Granzymes. The perforins produce channels in the infected cell's membrane. The granzymes enter through these channels and induce apoptosis in the infected cell.

MHC I

found on all nucleated cells (all mammalian cells) - displays epitopes on surface of cell **It allows differentiation of SELF from other (non-self). It displays short segments of the endogenous proteins found in the cell. These will be the same in all the cells in an individual, tagging the cell as belonging to "self". (when looking for organ donors, look for someone with the same MHC I to reduce immune response) ***The epitopes are loaded into a groove on the MHC I surface in the rER. The membrane associated molecules are then transported through the Golgi apparatus directly to the cell surface via vesicles

Follicular Dendritic Cells

organize lymphoid follicles They support the process of tuning the immune system that occurs there. During clonal expansion, the B lymphocytes actually mutate the DNA coding for the amino end of the light chain where the recognition of the antigen occurs, a process called hypermutation. As a result, some B cells will produce even better, more specific, antibodies. The dendritic cells then help these more effective clones to continue to multiply and they suppress the less effective ones.

killer-inhibiting receptors

recognize major histocompatability complex I (MHC I) on cell membranes In response to viral infection and as a part of tumor formation, cells often lose the MHC I on their plasma membrane. This attracts the deadly attention of natural killer cells.

***what does Helper T cell recognize on a macrophage and causes it to do?

recognize the MHC II on the macrophage stabilized by CD 4 T helper cell will then express the Interleukin 2 receptors on the membrane, and it will also secrete IL2 to AUTOSTIMULATE itself.

Effects of Immunity

response to a second exposure that activates the memory cells not only begins much sooner, 2 days instead of about 5, it produces far more antibody (x4) and reaches a peak sooner- about a week out immunized individual usually has a much less severe infection than a naïve individual titer is still high out past a month. This is why boosters give particularly good protection for the ensuing couple of months- there is still a lot of circulating antibody.

B Lymphocyte subclass

•Natural Killer Cells (already discussed) •Naïve Cells: Lymphocyes that have not yet met their specific antigen. •Effector Cells: Lymphocytes that have been activated by meeting their specific antigen, which **turn into plasma cells and make antibodies. •Memory B Cells: Activated lymphocytes that survive for many years providing immunity (immunological memory) and the ability to mount a secondary immune response.

T Lymphocyte subclass

•T Helper Cells: These cells interact with B lymphocytes, macrophages, neutrophils and other antigen presenting cells to both facilitate the immune response, and make sure that it is not initiated inappropriately. •Cytotoxic T Cells: These cells attack cells that have been infected with viruses, or are cancerous, or belong to invading organisms. •Memory T Cells: Like memory B cells, these cells survive for years and provide immunity against future infections. •Regulatory T Cells: Modulate the activity of other lymphocytes.