B Cell Activation, Affinity, Maturation & Class Switching Flashcards
When does a B cell become a mature B cell?
β When it is capable of making IgM and IgD
Once a mature circulating B cell encounters a pathogen what does it do?
β Migrated to the germinal center in the lymph node
Where does the mature B cell undergo affinity maturation and class switching?
β In the germinal center
What is the function of affinity maturation and class switching?
β Codes the heavy chain constant region of the B cell
What does affinity maturation do?
β further hones the variable fragment of the antibody to that particular pathogen
During affinity maturation what signals does the B cell receive and why?
β Receives signals from T cells about which pathogen the body is dealing with
β this is so it switches its heavy chain constant region to deal with the pathogen
What are the two things a B cell can differentiate into?
β Plasma cells
β Memory B cells
What is an antibody made up of?
β Two heavy chains and two light chains
What part of the antibody makes up the antigen binding variable fragment?
β The first domains of the light and heavy chain
What is the constant region of the antibody for?
β Biological effector functions
What is neutralisation and what does it do?
β Antibodies binding to the docking site of the virus
β It prevents the virus from entering the cells
β binds to the active site of toxins and neutralises the toxins
What is opsonisation used against and why?
β mostly bacteria because they are larger than viruses
How is opsonisation done?
β The antibody tags the surface of bacteria to recruit other cells such as macrophages
What happens once antibodies have opsonised a bacterium?
β The macrophage has Fc receptors on itβs membrane that bind to the CH2 region of the antibody
β The macrophage performs ADCP (antibody dependent cellular phagocytosis) for bacteria
How does opsonisation for tumor cells occur and what happens as a result of this?
β The antibody tags the tumor cell and recruits NK cells to perform ADCC (antibody dependent cellular cytotoxicity)
β NK cells produce chemicals to cause the tumor to apoptose because they are too large to be engulfed
What are the 4 functions of antibodies?
β Neutralisation
β Opsonisation
β Opsonisation for tumor cells
β Complement fixing
What are the two immune complexes formed by antibodies?
β A lot antibodies can bind to the surface of the bacteria and cause agglutination which gets cleared by macrophages
β complement molecules such as C1Q, C1s and c1r bind to the antibody and lead to phagocytosis and inflammation
What are the two ways complement fixing works?
β formation of MAC (membrane attack complexes) which punches holes in the tumor cells -lysing them
β antibodies can also form complexes with the complement (complement fixing) which leads to phagocytosis
How many classes of antibodies are there?
β 5
What determines antibody class?
β the difference in the heavy chain
What are 3 properties of IgG?
β 4 heavy chain domains
β stabilised by two disulfide bonds in between the heavy chains
What are disulfide bonds formed between?
β Two cysteine residues of amino acid
How is IgD different from IgG?
β it has a longer hinge that is stabilised by one disulfide bond
How many domains does IgE have?
β 5 domains
How is IgA different to IgG?
β similar but the third CH3 domain has a modification which allows interaction with the J chain so the two monomers can join together
β wrapped by secretory components to be secreted into the mucus
What is IgM similar to?
β IgD
How many domains does IgM have and how can it form a pentamer?
β 5 domains
β forms a pentamer with the help of the J chain
What is the function of IgM?
β Fixes complement
What is the heavy chain of IgM?
β Mu
What is the heavy chain of IgD?
β delta
What is the heavy chain of IgG?
β gamma
What is the heavy chain of IgA?
β alpha
What is the heavy chain of IgE?
β epsilon
Which Ig has the heaviest molecular weight and why?
β IgM
β it forms a pentamer
Which 2 Igs fix complement?
β IgM
β IgG
Which Ig crosses the placenta?
β IgG
What does the Fc on IgG bind to?
β phagocytes
What does the Fc on IgE bind to?
β mast cells
β Basophils
What is the function of IgM?
β forms immune complexes and fixes complement
β monomer serves as BCR
What is the function of IgD?
β BCR that indicates mature B cells
What is the only antibody that is not secreted?
β IgD
What is the function of IgG?
β Neutralises toxins
β opsonisation
What is the main antibody of the primary response?
β IgM
What is the main antibody of the secondary response?
β IgG
Where is IgA secreted into?
β mucus
β tears
β saliva
β colostrum
What is the function of IgE?
β allergy
β anti-parasites
What is the default Ig produced by B cells?
β IgM
Describe how B cells differentiate into plasma and memory cells from VDJ recombination?
1) after VDJ recombination of B cells they become mature circulating B cells
2) the lymphoid progenitor stem cell in the bone marrow gives rise to T cells
3) The T cells migrate to the thymus and undergo similar VDJ recombination to form TCR
4) the T cells receive signals from APC and become activated and activate B cells when pathogens are there
5) the B cell migrates into the germinal center for affinity maturation
6) in the dark zone clonal expansion occurs, then somatic hypermutation
7) selection occurs in the light zone
8) the B cells undergo the cycle between the light and dark zone many times and they undergo class switching
9) then they differentiate to plasma and memory cells and circulate
What is affinity maturation?
β honing of the variable fragment to the particular pathogen
What occurs in the dark zone?
β Clonal expansion
β Somatic hypermutation
What is somatic hypermutation?
β an enzyme creating mutations in the DNA
What is clonal expansion?
β the B cell makes clones of itself
What occurs in the light zone?
β Selection
Where are the light and dark zone found?
β Germinal center in the spleen or lymph node
What is the cycle through the light and dark zone called?
β Affinity maturation
What are the two stages of B cell activation?
β T cell independent
β T cell dependent
What happens in the T cell independent stage?
β When the B cell encounters the antigens it undergoes clonal expansion
β Some of them differentiate into IgM secreting plasma cells
What happens in the T cell dependent stage?
β Some B cell clones migrate into the lymph nodes
β The B cell needs to recognise the antigens, internalise them and then present them with MHC class II
β the B cell needs to be activated by the TCR (the T cell has to have been activated by the same pathogen)
β The CD40 and CD40L join
β The T helper cell produces cytokines to complete the 3 step verification process
What does differentiation and clonal expansion of activated B cells require?
β Antigen binding to BCR
β co-stimulation by activated Th cell specific to the same antigen
β Th cell-derived cytokines
How many unique B cells does the VDJ recombination produce?
β 1 billion B cells with unique B cell receptors
How does clonal expansion occur?
β When a pathogen invades the body the B cell receptor with the closest binding to the pathogen gets activated
β it will make more clones of itself
β The clones migrate to the germinal center for affinity maturation
Describe affinity maturation?
β the naive B cell that has been activated by a T cell enters the dark zone of the germinal center and undergoes clonal expansion
β the enzyme AID makes point mutations in the variable regions of B cells at random points
β after AID the B cells are all unique - somatic hypermutation
β the B cells migrate into the light zone to undergo selection
β the FDC presents the antigens on its surface
β the B cells will compete to bind to the FDC
β the B cells with high affinity bind and present the antigens to the Tfh cell which sends a survival signal
β B cells with reduced affinity undergo apoptosis because there is no survival signal from Tfh
β this is repeated for several cycles until the affinity is high enough
Where does affinity maturation occur?
β In the germinal center at the periphery of the T cell zone
What two types of cell assist with affinity maturation?
β Follicular dendritic cells
β T follicular helper cells
What is the function of follicular dendritic cells?
β They stay in the germinal center and present antigens for the selection process
β not normal dendritic cells
What can T follicular helper cells do that other T cells cannot?
β Enter the germinal center
How is major heavy chain class switching done?
β DNA recombination
How is minor heavy chain class switching done?
β mRNA differential splicing
What is major heavy chain class switching?
β IgM to IgG, IgA and IgE
β IgG to IgA and IgE
What is minor class switching between?
β IgM and IgD
What three cytokine signals are needed for IgG1 and what cells produce them?
β Increased IL-4 - TH2
β Decreased IFN gamma - TH1
β Increased IL-10 - Treg
What 4 cytokine signals are needed for IgG3 and what cells produce them?
β decreased IL-4 - TH2
β Increased IFN gamma - TH1
β Decreased TGF beta - Treg
β increased IL-10 - Treg
What cytokine signal is needed for IgG4 and what cell produces it?
β increased IL-4 - TH2
What two cytokine signals are needed for IgA and what cells produce it?
β increased IL-5 - TH2
β Increased TGF beta - Treg
What two cytokine signals are needed for IgE and what cells produce it?
β Increased IL-4 - TH2
β decreased IFN gamma - TH1
How does cytokine signalling occur?
β CD40L on T cells interacts with CD40 on B cells
What are the 3 steps for class switch recombination to occur?
1) cytokine signal
2) switch signals
3) AID and DSB repair proteins
Where does class switching proceed and why?
β Only downstream because the segments are removed
β you cannot switch to something βbehindβ you
What can IgM switch to?
β IgG
β IgA
β IgE
What can IgG switch to?
β IgA
β IgE
How are unneeded classes cut out?
β Enzymes turn the unneeded segments into loops and they are cut out
β the mRNA no longer contains the other regions
What is the difference between membrane bound and secreted Ig?
β the secreted form has a tail piece
β the membrane bound has an anchor made up of a hydrophobic transmembrane region and a cytoplasmic tail
Describe how differential splicing of ΞΌ constant regions occurs?
β the VDJ is not changed
β the C ΞΌ region is made up of ΞΌ1, ΞΌ2, ΞΌ3 and ΞΌ4, a secreted region, a stop codon, a poly A site
β followed by a membrane M1, M2 region, a stop codon and a poly-A site
β for secreted heavy chains the DNA region is transcribed and spliced at the first poly-A site so the membrane regions are not transcribed
What does BCR binding lead to?
β activation of tyrosine kinase
β signal transduction pathway for cell proliferation, differentiation and survival
What is the BCR that initially binds the antigen called?
β a low affinity antibody
