Lecture 14 Objectives
What is the lymphatic system (lymph, lymphatic vessels, lymphoid organs)?
1) Lymphatic System: Surveillance system that screens the body for foreign molecules and where Adaptive Immune Cells hang out, produce, and mature, and interact to modulate responses. 2) Lymph-Watery Liquid with similar composition to blood plasma-Arises from fluid leaked from blood vessels into surrounding tissues, into intercellular spaces. This will carry pathogens and toxins to ares where lymphocytes are concentrated. Flows one way. 3) Lymphatic vessels-One-way system that carries lymph from tissues and towards the heart and returns it to the circulatory system-Lymphoid organs 4) Lymph Nodes - Located at various points of lymphatic vessels. They house lots of Leukocytes, especially Lymphocytes. They will recognize and attack any foreign invaders present in Lymph. They facilitate interactions among immune cells and then any material arriving at Lymph Nodes. 4) Primary lymphoid organs-Red bone marrow for B cells-Thymus for T cells. Where Lymphocytes mature. When they are ready they go to Secondary. 5) Secondary lymphoid organs-Lymph nodes-Spleen-Tonsils-Mucosa-associated lymphoid tissue (MALT). Here they wait for foreign microbes
Explain four mechanisms by which antibodies can help eliminate pathogens.
1)Neutralization: Antibodies can neutralize a toxin by binding to a critical portion of the toxin. Similarly, antibodies can block adhesion molecules on the surface of a pathogen. Ex: A pathogen has a toxic part that has to bind to a receptor on one of our cells in order to enrer. An Antibody can bind to that portion 2)Opsonization: Can bind to pathogen, making it easier for phagocytes to engulf the pathogen. Ex: Variable portion of the antibody binds to the epitope on the pathogen. This is tagging it for phagocytes and makes it easier for them because the Phagocyte has receptors for the FC portion of the antibody = makes it more attractive. 3) Agglutination: Numerous antibodies can aggregate antigens together-increases chances they will be filtered out of blood. Because antibodies have 2 identical antigen binding sites. So it can cluster microbes together and easier for them to be filtered out or be eaten up. 4) Antibody-dependent cellular cytotoxicity (ADCC): antibodies coat pathogen, Natural Killer cells bind to the constant region of the antibody and release toxic (perforins/granzymes) stuff that kills the pathogen.
[Big Picture, Connection - L15] How do the roles of MHC I and MHC II differ in the immune response? HINT: Consider humoral vs. cell-mediated immunity.
MHC I - cell mediated immunity, all nucleated cells have MHC I to display self or non self antigens, Tc detect and destroy non self4 MHC II - only in humoral B cells and macrophages, only display pathogenic antigens.
What consequences would down-regulating MHC2 on macrophages have on the immune system?
Macrophages are antigen presenting cells. if MHC2 is downregulated that means it would be a poor presenter of epitopes to T-Cells so that would diminish production of Helper T Cells and that is going to interfere with both humoral and cell mediated immunity.
[Big Picture] Can each B cell make many different antibodies, or only one antibody? Are there B cells in our bodies that make antibodies that match 'unseen' antigens? Differentiate between the following terms: lymphocyte, B cell, naïve B cell, plasma cell, memory cell.
Many different B Cell Receptors can recognize any particular antigen but Each B cell receptor only one Epitope of that Antigen. Each B cell produces a single species of antibody, each with a unique antigen-binding site. When a naïve or memory B cell is activated by antigen (with the aid of a helper T cell), it proliferates and differentiates into an antibody-secreting effector cell (plasma cell) So once a B cell binds to an epitope, it starts to clone itself. Some of them are going to be Memory B Cells which are cells that stick around for a long time with a perfect variable portion of their receptor. These will be in higher quantities than there were originally in case it encounters the same pathogen 10 years in the future. The others will become Plasma or Effector Cells. They are antibody factories. They will produce those same antibodies with that perfect varible and start spitting them out. So if a virus enters those antibodies will bind to it and that will essentially tag them and makes it easier for phagocytes to eat them up = called Opsonization. Lypmhocytes: Lymphocytes are white blood cells that are also one of the body's main types of immune cells. They are made in the bone marrow and found in the blood and lymph tissue. - NK Cells - Destroy target cells by cytolysis - B Cells - Produce antibodies - T Cells - Cell-mediated immunity ** During B Cell development, there is a lot of shuffling of the portion of their DNA that codes for parts of these proteins (antibodies) which leads to the diversity in the variable portions Cytotoxic T cells scan the surface of cells in the body to see if they have become infected with germs, or if they have turned cancerous. If so, they kill these cells.
Explain how memory cells help to speed up future immune responses to pathogens the body has already seen. Distinguish a primary from a secondary immune response.
Memory B Cells are long lived that are readily activated upon secondary exposure to the antigen. Plasma cells pump out antibodies, but individual plasma cells die. Memory B cells can survive for decades and repeatedly generate an accelerated and robust antibody-mediated immune response in the case of re-infection (also known as a secondary immune response). During an immune response, B and T cells create memory cells. These are clones of the specific B and T cells that remain in the body, holding information about each threat the body has been exposed to! This gives our immune system memory.
Memory Cells
Memory cells are produced by B Cells proliferating but they don't secrete antibodies Memory Cells and the Establishment of Immunological Memory -Memory cells: Produced by B cell proliferation but do not secrete antibodies -Have B Cell Receptors complementary to the epitope that triggered their production -Long-lived cells that persist in the lymphoid tissue -Initiate antibody production quickly if antigen is encountered again Primary immune response• 1) When the antigen first binds to the B Cell, activated, and proliferates. Clonal Selection is going to happen that gives rise to plasma cells and memory cells. In this response there are just a few helper T cells and B cells that can recognize the antigen 2) SO Small amounts of antibodies produced and takes days before sufficient antibodies are produced -Serum contains no detectable antibodies for 4-7 days. IgM produced, then IgG, peaking in ~10-17 days. -May take days to produce enough antibodies to eliminate the antigen from the body Secondary immune response - Mostly IgG. peaking in ~2-7 days 1) Memory cells become active/respond to another exposure to the antigen 2)Antigen presence directly stimulates the memory cells to become plasma cells. (No T Cell needed) Rapidly differentiate into plasma cells -Much faster and greater in magnitude than the primary response - A Third exposure will result in an even more effective response. 3) This response: most cells will switch from IgM to IgG as they become plasma cells. 4) Hypermutation: of the variable region. By mutating the variable region this can increase or decrease the affinity of the antibody for its antigen - what eventually happens is the higher affinity antibodies are selected because as the antigen becomes more scarce over time, the immune response kicks in, only those antibodies with higher affinities can bind and become activated. Generating these antibodies with a higher affinity for their epitope results in a more effective immune response. -Mostly IgG. peaking in ~2-7 days Antibody titer: relative amount of antibody in the blood serum.
What is the function of MHC? Which cells make MHC I, and which make MHC II? What two WBC classes can present antigens in MHC II? How do these classes differ in the number of different antigen types they can present?
Presenting Antigens to T-Cells 2 types MCH class I and MCH class II MCH class I: found in the nucleated cells. (All of the cells except RBC.) CD8 CYtotoxic are attracted to MCH1. Typically Can display epitopes of intracellular pathogens. Ex: virus Can display a piece of that virus as a tag or Marker, indicating to a T-cell " I am infected come destroy me" MCH class II: only found in antigen presenting cells (APC's)APC's-Macrophages, dendritic cells , B-cells. APC degrade microbial pathogens and present distinct pieces on their MCH proteins to T Helper CD4 cells in order to mount and immune response against that antigen. They engulf a pathogen via phagocytosis, chew them up, and get pieces and present distinct pieces on their MCH protein to APC, this helps them mount an immune response against that pathogen. MCHII can display extracellular pathogens floating around. Brings them in via cytosis and displays those to T-cells. MHC Proteins Function: Major Histocompatibility Complex (MHC) Proteins hold and display antigens for presentation to T cells. -In cytoplasmic membrane T cells can only recognize epitopes pieces that are bound to MHC molecules.• **2 Types: MHC Class I and MHC Class II MHC Class I: Found on all nucleated cells except RBCs. Very broad in their binding and can bind to many different type of Epitopes MHC Class II: Only found on antigen presenting cells (APCs).-Macrophages, dendritic cells, B cells. APCs degrade microbial pathogens and present distinct pieces on their MHC proteins to T cells in order to mount an immune response against that antigen. These will do Phagocytosis/ or receptor mediated endocytosis. Engulf the pathogen, chew up, and present distinct pieces on their MHC proteins to T Cells, and a mounted immune response on that pathogen Antigens that come from intracellular pathogens are displayed on MHC I molecules.-See lecture 15.•Antigen-Presenting cells phagocytose an extracellular pathogen, chew it up, and display the antigens on MHC II molecules.
Adaptive Immunity Attributes
The body's ability to recognize and defend itself against distinct invaders and their products. Tailored reactions against specific invaders.. Ex: AI can distinguish between friendly E. coli and E. coli157 which is pathogenic Five attributes of adaptive immunity: 1) Specificity : Any particular adaptive immune response acts against only ONE particular molecular shape 2) Inducibility : Cells of adaptive immunity are activated in response to specific pathogens. The presence of a pathogen is going to activate or induce the cells. .3) Clonality : Once induced, cells proliferate (many clones are produced) They form generations of identical to nearly identical cells. 4) Unresponsiveness to self: Adaptive immune responses must be self-tolerant! There are many mechanisms in place to ensure immune responses are not mounted against your cells if something goes wrong (Autoimmune diseases could occur) 5) Memory: Adaptive immune cells can recognize pathogens that have infected you in the past upon exposure. Responses have memory for specific pathogens- meaning it adapts to respond faster and more effectively in subsequent encounters with the invader to destroy it. Involves activity of lymphocytes -B lymphocytes (B cells): Produce ANTIBODIES •Mature in the bone marrow -T lymphocytes (T cells): assist other WBCs, destroy infected cells Mature in the thymus
[Connection - L4] How does your knowledge of protein structure (primary, secondary, tertiary, quaternary) relate to the specificity of an antibody? Do antibodies have quaternary structure?
The specificity of the protein's shape in the variable region dictates which antigens the antibody can bind to. The disulfide bridges mean the antibodies have quaternary structure.
What is class switching? Why does this occur? What part of the antibody changes? Know the 5 classes and major features of each as discussed in class.
A person ususally has 5 different classes of antibodies defines by 5 different types of heavy chains: IgM, IgG, IgA, IgD, and IgE . They all have different functions. 1) All antibodies start off as IgM. The B Cell Receptor is IgM. In a process called class switching, the Fc portion of an antibody is changed. A plasma cell combines its variable region gene with a different constant region gene, resulting in a new antibody class with different functions. Classes of antibodies• 1) IgM—first antibody produced during the early stages of an immune response. It is the first detected, but exists in a circulating form and exists in a pentamer. A bid molecule. It has 5 antibodies, 10 binding sites. This is not able to cross the placenta. Involved in activating complement, agglutination, and neutralization. *IgM acts as a B Cell Receptor 2) IgG—most common, abundant, and longest-lasting antibody. Seen in blood and tissue fluids. This can do opsonization, neutralization, activate complement. It has many functions: because it can enter extracellular spaces more easily than other antibodies. Only antibody that can cross the placenta. 3) IgA—associated with body secretions. Secreted across mucosal surfaces. It can do agglutination, neutralization, it can exist as a dimer. It usually exists as a dimer. Found in the breast milk (can protect baby), tears, saliva, mucous. Main function is preventing attachment of pathogens to mucosal surfaces, pathogens that use mucous membranes as a portal of entry 4) IgE—involved in response to parasitic infections and allergies. Found on the surface of Eosinophils and mast cells (venus fly trap) sits and waits. When it binds its antigen, a signal is sent into the cell and then it triggers a histamine to be release and gives rise to inflammation, anafalaxis, other hypersensitivities. Triggers release of molecules from Eosinophils, anti-parasitic molecules from Eosinophils, and histamine from basophils and mast cells. 5) IgD—exact function is not known
[Big Picture] What 2 things MUST occur to activate a B cell? What is the advantage of using 2 'checkpoints' (rather than just one)?
APC will bind a microbe and bring it in via Phagocytosis , lysosome comes and now its a phagolysosome. The microbes degraded into pieces which are epitopes. Epitopes will form a complex with MHC class 2. The APC will (with the epitopes) will go to lymph nodes where lymphocytes hang out.. A Helper T Cell (CD4 protein) with a T cell receptor that is specific for this MHC antigen complex. That will bind and then then the T-cell is activated. It will go look for a B cell that has encountered the same antigen. Naive B cell hasnt encountered it's the same antigen. This B cell is a Naive B cell and the same antigen will bind to the B cell receptor. It doesn't have to recognize the same epitope as the antigen presenting cell or the T cell recognized but it is the same antigen. This is the first signal to activate the B Cell and initiate its immune response. The B cell internalizes the microbe and the receptor gets recycled as well. The microbe is degraded because B cells are antigen presenting cells also and will display the antigen and MHC II. The Helper T Cell then binds to this. This is the second signal that the Bcell needs to activate. Now the B cell can proliferate: called Colonal Expansion. It will replicate and most cells will become Plasma cells which are antibody making factories that pump out huge number of antibodies. Others will become Memory B Cells. Memory B Cells can quickly generate. Plasma cells upon exposure to the same antigen months or even years later can remember that antigen and can quickly initiate a response. Its not enough for Bcell to just bind an antigen with its B receptor and then its activated. there are all these steps that must occur.. It has to interact with T Cell and T Cell gives permission to B Cell to generate Plasma Cells. Imagine a B Cell with its receptor that could bind to one of your SELF antigens, that would be bad if that alone was suffcient to cause the B Cell to initiate the strong immune response. The Second signal will prevent inapproproite immune responses from happening.
Differentiate humoral immunity and cell mediated immunity- what are the main immune cells involved, and what types of pathogens are targeted?
Adaptive Immunity has 2 branches: targets parts of microbes: antigens 1) Humoral Immunity - Antibody (humoral) immune responses- carried out by B cells (but T cells play a role too). Often targets or act against extracellular pathogens or toxins those not inside of our cells 2) Cell-mediated immune responses- carried out by T cells. Often act against intracellular pathogens. Ex: Virus gets in host cell and takes over, cell mediated will target these intercellular pathogens
Define: antigen, epitope, hapten.
Adaptive Immunity is not directed at entire microbe but rather parts of them, which are Antigens. 1) Antigens: Molecules the body recognizes as foreign and worthy of attack. Ex: parts of cell wall, pili, flagella. - Antigens are Recognized by three-dimensional regions called epitopes. - Large, complex macromolecules make the best antigens. Include proteins and glycoproteins which are most effective. Carbs and lipids can be antigenic. Include various bacterial components as well as specifically proteins of viruses, fungi, and protozoa Food, pollen, dander, and other "innocuous" molecules can be antigenic 2) Epitopes: Antigenic determinants. These are on the antigen, they make up the antigen. They are specifically recognized by the immune cell. 3) Hapten: Too small to be antigenic on their own. Substance with a low molecular mass that is not antigenic unless it is attached to a larger carrier molecule. Ex: Penicillin. It is too small to trigger immune response, but bound to larger protein in the blood can trigger allergic response
Distinguish between antibodies and antigens. Be able to draw an antibody and label the variable (antigen-binding) region and the control region. What results in the specificity of an antibody (i.e. one antibody can bind only one particular antigen)? Which type o f lymphocyte makes antibodies?
An antigen is any substance or organism that is unrecognized by our immune system. It could be anything from bacteria to chemicals, to viruses ... or even foods [1]. Antigens typically trigger an immune response, which may consist of an antibody (more on that later), and are classified by their origins Antibodies are proteins that bind with the antigen in order to neutralize the latter - or make other elements of the immune system "aware" of their presence. Antibody-producing cells are specifically designed to tackle one type of antigen; and your blood, bone marrow, lymph glands, and spleen will contain millions of them to ensure that every known antigen will be confronted by a corresponding antibody Antibodies are secreted by B leukocytes (a form of white blood cell) and circulate in blood plasma either freely or attached to the surface of a B cell. The B and T cells work in unison to identify and locate antigens, create the correct antibodies, and capture (kill/neutralize) the antigen [6]. A vaccine, by exposing the immune system to a new antigen, will "teach" antibodies the correct format in which to capture or tag that antigen. When the actual disease antigen later enters the body, the immune system will rapidly respond with minimal discomfort and inconvenience. When the BCR is released from the B Cell and is no longer attached to the membrane and is free-floating, it is called an ANTIBODY (AKA immunoglobulin). Any antibody will circulate thru blood and lymph and ignore all antigens except the target antigen. When it binds to antigen, several things happen that leads to killing it. Targets for destruction. The surface of each B lymphocyte is covered with ~500,000 identical BCRs unique to that cell. All specific to one Epitope and all recognize the same thing. 10^9 or 10^13 B cells each with its own unique B cell receptors
What is clonal selection? Why do activated B and T cells differentiate into mostly plasma (or effector) cells, with a smaller fraction becoming memory cells.
Clonal Selection is the process that gives rise to Plasma Cells and Memory Cells Each B Cell has receptors that have a specific shaped variable region (different than other B Cells) which produces an antibody that is specific for one epitope. 1) Stem cells differentiate into mature B cells, each bearing surface immunoglobulins against a specific antigen. 2)B cell III complexes with its specific antigen and proliferates. 3) Some B cells proliferate into long-lived memory cells, which at a later date can be stimulated to become antibody-producing plasma cells. Memory cellsB cell III complexes with its specific antigen and proliferates. 4)Other B cells proliferate into antibody-producing plasma cells. 5) Plasma cells secrete antibodies into circulation. Antigens in circulation now attached to circulating antibodies.
Cytokins
Cytokine production, Signal transduction leads to changes in gene expression.- 1) Interleukins (ILs)•Signal among leukocytes (There are 35 different interleukins)- 2) Interferons (IFNs)•Inhibit the spread of viral infections and act as cytokines. IFN-gamma, a phagocytic activator 3) Growth factors•Proteins that stimulate stem cells to divide 4) Tumor necrosis factor (TNF)•Secreted by macrophages and T cells to kill tumor cells and regulate immune responses and inflammation 5)Chemokines•Chemotactic cytokines that signal leukocytes to move to site of infection/inflammation How do cells get activated: Cytokines are produced by Helpter T's and act as signaling molecules. They bind to receptors and change gene expression = signal transduction. When these cells are activated they are signed to proliferate. Helper T's will proliferate and create cytokines
Compare and contrast B cell and T cell receptor structure and function. How do they recognize epitopes?
How can antigens (specifically, epitopes) be recognized so an immune response can be mounted?•Lymphocytes (B cells and T cells) have receptors in the cytoplasmic membrane that bind antigens.-These function very differently! 1) B cell Receptor (BCR) - A Y-shaped protein (glycoprotein) It is composed of 4 polypeptides. 2 light chains and 2 heavy chains. Held together by disulfide bonds. -2 identical antigen-binding sites on one B cell receptor called Variable region: binds to the epitope, varies among B- cells. Variable region will directly interact with Epitope - constant (Fc) region: determines what class of antibody- IgG, IgM, IgA 5 diff types of heavy chains (more on this later). Various types of Constant Regions impart distinct purposes to each class of anitbody. The surface of each B lymphocyte is covered with ~500,000 identical BCRs unique to that cell. 2) T-Cell Receptors: Also comprised of protein with constant and variable regions that recognize a specific epitope... but differ from BCRs in an important way:• TCRs do not recognize and bind antigen or epitopes directly, but instead recognize short peptide fragments of antigens which are bound to MHC molecules on the surfaces of other cells. A B Cell receptor is recognizing the whole 3 D like structure of its epitope but the T Cell receptor can only bind to fragments of an Epitope that are bound to surface molecules on some of our cells
Give a detailed account of the humoral immune response. Be sure to address the roles of Antigen presenting cells, helper T cells, and B cells. Compare and contrast T-dependent and T-independent antigens.
Humoral Immunity is where the body induces antibody immune responses against antigens of extracellular pathogens and toxins. Two types of Antigens: T Dependent: A response against T-dependent antigen requires the help of a Helper T cell. Most anti-body responses operate this way. Ex: T Dependent Antibody Immune Response: Antigen presenting cell (like dendritic) and an extracellular microbe. 1) APC will bind the microbe and bring it on via phagocytosis. 2) Its in the phagosome, lysosome combines, and its phagolysosome. 3) Microbe is degraded into pieces. The pieces are epitopes that will form a complex with MHC class 2. 4) APC will travel the lymphatic system and go to lymph nodes where lymphocytes hang out. 5) Helper T cell with CD4 protein T-Independent: No helper T cell needed. 1) This tends to be the case for poly or lippo poly saacharides. 2) Ex: Components of capsules or the S layer are antigens that that consist of repeating subunits that can cross link several B-Cell receptors on a B cell. When the receptors are simultaneously recognizing that epitope it can lead to an immune response. 3) It is not as robust of a response and does not lead to immunological memory. •Does not require help from T cells•T-independent antigens tend to consist of repeating subunits-Repeating units can crosslink several BCRs on a B cell
[Big Picture] Explain why humoral immunity alone is not enough to combat viralinfections. At which stage (if any) in the viral life cycle is humoral immunity effective?
Humoral immunity alone is not enough to combat viral infections because it only works when the virus is outside of the cell, which is not very long so there is not enough time for it to take place.
Briefly explain how lymphocyte receptors can be diverse enough to recognize billions of antigens, despite the fact that they are encoded by relatively few genes.
•Do you think there is an individual gene for every receptor?•-(No!! That would be a lotttttt of DNA.)•Basically, these genes are chopped up and rearranged, and nucleotides are added and deleted at random to create diversity at these loci. (If you're interested, look up VDJ recombination).•You have 1023 B cell receptor possibilities. That is a number 10 times greater than all the stars in the universe!!!!!!!!!!!!