topic 6 Flashcards
What is the general process of b cell activation? What are the two different types of b cells?
Antigen is recognized
B cell is activated (maybe by Th cell or by other stimuli)
Clonal expansion
Differentiation-
Antibody secretion, class switching, affinity maturation, memory b cells
What are the different subsets of b cells and what do they respond to and how?
Follicular b cells respond to protein antigen and helper t cells (in germinal center) and leads to various isotypes (igG, IgA, igE) and long lived plasma cells
Marginal Zone B cells are in the spleen, respond to polysaccharides, lipids, etc. and lead to IgM and short lived plasma cells
B-1 B cells are found in the mucosa, respond to polysaccharides, lipids, etc. and lead to IgM and short lived plasma cells
What are some differences between primary and secondary antibody response? What are the different antibodies involved in each? How does the antibody effect differ in both?
Primary-peak response smaller, igM>igG, lower average antibody affinity
secondary-peak response larger, igG has a relative increase and sometimes igA or igE are involved (heavy chain class switching. There is a higher average antibody affinity (affinity maturation).
What are some specific steps of antigen recognition and b cell activation?
Ags are delivered to primary follicles of peripheral LN via lymphatics or also by macrophages or DCs. In the spleen they can be delivered to the b cells via blood, or in DC or macrophages
The Ag is recognized using Ig receptors.
When the Ag forms an antibody, Ig-alpha and Ig-beta are brought into close proximity to form the BCR complex which then undergoes tyrosine phosphorylation through its ITAM
This leads to a signalling cascade which results in transcription factors being formed which leads to proliferation and differentiation.
How do complement (C) and TLRs play a role in b cell activation?
C binds to microbe which leads to a signal cascade resulting in activation.
TLR binds to a PAMP from the microbe and then sends a signal leading to activation.
How does activation differ between Ti and Td Ag
For Ti Ag, the activation results directly in proliferation and differentiation and thus more crosslinking of receptors occurs and C’ is activated more strongly.
For Td Ag, the activation simply prepares the cells to be fully activated by Th
How are Td’s prepared for Th’s?
Activation of these b cells leads to clonal expansion, increased expression of cytokine receptors, decreased chemokine receptors (migration to t cells), and secretion of low levels of IgM.
How and where do b cells meet?
B cells lose chemokine receptors and migrate from follicle into t cell zones.
When b cells bind with an antigen, they phagocytose it and cut it up and present it on the membrane in a class II MHC. The T cell recognizes to epitope/MHC.
T/B cells enter into a germinal center in the lymphoid follicle
After recognition, how do Th cells activate b cells?
Upon binding, CD40 on the b cell recognizes the CD40 ligand (expressed upon binding) on the t cell. When they bind, the t cell releases cytokines which are recognized by cytokine receptors on b cells. This leads to differentiation and proliferation.
What is heavy chain class switching? Which cytokines lead to b cell proliferation? Which cytokines lead to the specific isotypes?
Heavy chain class switching is how different isotypes are accomplished. Different cytokines cause different receptors (ABs) to be expressed.
IL-2,4,5 lead to be cell proliferation in general.
IL-2,4,5 lead to IgM
IL-4,6,2 and IFN-gamma lead to IgG
IL-5 and TGF-beta lead to IgA
IL-4 leads to IgE
What is the molecular/genetic mechanism for class switching?
Activation induced deaminase causes differential splicing which leads to different isotypes being transcribed.
How they are spliced depends on which cytokines are received.
What is the main factor that determines which isotype is needed?
It is largely based on site. Where a b cell will dwell determines which function is needed most and thus determines the isotype.
How does Affinity maturation work?
Once the Th/B cells are into the germinal center, proliferation begins, and random mutations occur very frequently in the CDR regions (areas in V gene). This is referred to as somatic hypermutation. Mutations that lead to a really good fit with the antigen are selected. Thus as time goes on, the cells bind to the antigen with much higher affinity.
How are high affinity b cells selected?
B cells that are able to recognize antigen are able to bind to the antigen on follicular dendritic cells and then present the antigen to helper t cells. B cells that can do this receive survival signals. B cells that have mutated the wrong way don’t receive survival signals and die.
What is the difference between t independent and dependent b cells
t dependent is what we studied in detail. In t independent, polymeric antigens are recognized because they can be recognized by multiple receptors on a b cell and thus cross-link receptors which leads to a fairly strong activation. They have little or no class switching and thus only IgM and some IgG and little or no affinity maturation. memory b cells are only produced when dealing with some antigens, not all.
How is the humoral response stopped?
2 Ig cells bind the same polyvalent antigen. Once Ig cell is also bound by an Fc receptor on a b cell membrane. This Ig crosslinking with Fc receptor coligation leads to a block in b cell receptor signalling.
What are the different effector functions of Ab
Neutralization
Opsonization–>phagocytosis
Ab dependent cellular cytotoxicity (NK cells)
complement activation–>lysis, opsonization, inflammation
What determines which effector function occurs? Which isotypes cause which functions?
The isotype determines the effector function (Fc)
IgG-neutralizaiton, opsonization, activation of classical pathway of complement, Ab dependent cellular cytotoxicity (NK cells), feedback inhibition of B cell activation
IgM-Activation of classical pathway of complement in blood.
IgA-Mucosal immunity-neutralization in mucosa
IgE-defense against helminths (tapeworms), mast cell degranulation
How does neutralization work?
Antibodies bind to microbes or toxins and prevent them from infecting cells. They can prevent a microbe from binding to a cell making it unable to infect it. It can also bind a microbe when it is released from a dead infected cell and make sure it doesn’t bind to an adjacent cell. It can also prevent toxins from binding to a receptor on a cell.
What are the steps of opsonization ?
IgG binds the microbe. Fc receptors on a macrophage bind the IgG Fc. this signals to the macrophage that it should phagocytose the microbe. The microbe is phagocytosed then killed.
What are the different Fc receptors? What cells are they on? What Ig’s do they recognize? What is their function?
Fc-gamma-RI (CD64) has a high affifinity for IgG cells. They are on macrophages, neutrophils and some eisinophils. Function is phagocytosis.
Fc-gamma-RIIB (CD32) have low affinity for IgG. They are on B lymphocytes and lead to feedbac inhibition of b cells
Fc-gamma-RIIIA (CD16) has a low affinity for IgG. It is on NK cells and leads to ADCC.
Fc-epsilon-RI has a high affinity for IgE. It is on mast cells, basophils, eosinophils and leads to cell activation (degranulation).
How does ADCC work? How does IgE work on helminths?
IgG binds to microbe. NK cells bind to Fc portion of IgG using CD16. NK cell kills microbe.
IgE binds to a helminth. Eosinophils or mast cells bind IgE using Fc-epsilon-RI.Helminth is killed.
What is complement? How is it activated? How does it work? What is its role?
It is a collection of circulating and cell membrane proteins that amplify the response of antibodies and are also involved in innated immunity.
Activation involves sequential proteolytic cleavage leading to different effector molecules
The cascade is greatly amplified. Molecules become tightly attached to the microbial cell surface so they stay in the right site. Because of this amplification, it must be highly regulated.
What are the 3 main goals of the complement system? How are they brought about (which proteins)?
Inflammation (chemotaxis)-C3a, C4a, C5a lead to the recruitment and activation of leukocytes which kill microbes.
Opsonization of microbes–>phagocytosis-C3b and C4b are recognized by macrophages which leads to phagocytosis.
Lysis of microbes via MAC-C3b leads to formation of MAC which leads to lysis.
