Immunology - Week 2 - Part II - B cells Flashcards
What are memory T cells?
Memory T cells differentiate from CD8 (more of these) and CD4 positive T cells and survive after the infection has been eradicated. They do not produce cytokines or kill infected cells, but retain these capabilities upon subsequent recognition of the same antigen that they recognize
What does the BCR need to do to get passed the FIRST checkpoint during maturation?
Express the variable and constant region of the heavy chain
What does the BCR need to do to get passed the SECOND checkpoint during maturation?
Express the variable and constant region of the light chain
What are the recombination events that lead to the heavy chain (IgH) of BCR?
1st recombination event: D and J recombination 2nd recombination event: V with DJ 3rd recombination event: VDJ with the Constant region mu Note: many different types of V, D and J
What are the recombination events that lead to the light chain (IgL) of BCR?
1st recombination event: V with J 2nd recombination event: VJ with the Constant region mu Note: many different types of V and J
Where do naive mature B cells take up residence?
Peripheral immune Tissues: Follicles of lymph, payers patch of ilium, MALT/SALT
What is the structure of BCR?
2 heavy chains, each with a variable and constant region. 2 light chains, each with a variable and constant region.
What does the FAB region consist of?
On BCR (antibodies): Variable region of the heavy chain (Vh) 1st part of constant region of heavy chain (Ch1) Variable region of light chain (VL) Constant region of light chain (CL)
What does the Fc region consist of?
The constant regions of the heavy chains that are not in proximity with the light chains.
What part of BCR binds the antigen? (2)
variable regions of the heavy and light chains
What antibodies are expressed first and second during B cell maturation? What constant region are each associated with? First? Second?
First: IgM, Cu Second: IgD, C(gamma)
What part of IgM and IgG are different on the same B Cell?
the constant FC region (constant regions of the heavy chains)
What is a plasma cell?
terminally differentiated B cell
How do B-cells recognize SMALL antigens? How? Where? From who?
How: Small antigens stick to cell surface and are not processed Where: follicle of peripheral lymph tissue from who: Follicular dendritic cells
How do B-cells recognize LARGE antigens? How? Where? From who?
How: large antigens stick to cell surface and are not processed Where: subcapsular sinus From who: macrophages (note: dendritic cells in the medulla also present stuck antigens)
What are follicular dendritic cells?
dendritic cells that stay in follicles cells and do not migrate out. Small antigens stick to them and are presented to B cells
What are Folllicular B Cells? Express? Where? Types of antigen responds to? Types of plasma cells? T
Express: IgD and IgM Where? Spleen or other lymphoid organs Antigens: Protein Plasma cells: isotype switching, high affinity antibodies, long lived
What are Marginal B Cells? Express? Where? Types of antigen responds to? Types of plasma cells? Types of
Express: IgM and IgD Where: Spleen and other lymphoid organs Types of antigen responds to? Lipids and polysaccharides Plasma cells: short lived, mainly IgM
What are B-1 Cells? Express? Where? Types of antigen responds to? Types of plasma cells? t
Express: IgM and IgD Where: payers patches (mucosal tissues and peritoneal cavity) Antigens: Lipids and polysacchardies Types of plasma cells? short lived, mainly IgM
What is the FIRST SIGNAL in B Cell activation?
2 Antigen molecules bind to BCR
What is the SECOND SIGNAL in B-Cell activation? (2 ways)
1) Compliment receptor (CR2 or CD21) on B-Cell is activated by the C3d compliment protein 2) TLR (toll-like receptor) on B-cell is activated by PAMP
What is CD19?
B-Cell marker
What is ITAM on B-cells? What? Where?
What: tyrosine-based activation motif Where: located on the accessory signal receptors (Iga and IgB)
What does activation of naive B-cells result in?
Clonal expansion
How do B-Cells differentiate into Effector B-Cells? (2)
1) T independent 2) T dependent
What is T-independent differentiation of B-cells? Description? Location? (4) Type of antigen?
Description: differentiation occurs in the absence of T cell help Location: spleen, bone marrow, peritoneal cavity, mucosal tissues Type of antigen: lipids or antigens with repetitive structures
What is T dependent differentiation of B-cells? Description? type of antigen?
Description: After B-cell activation by antigen, mother B-cell migrates outside of follicle cell and presents antigen via MHC class II molecule to effector T cells. B-cell makes clones and they migrate back to follicle. Type of antigen: proteins
What do B-cells who have recognized an antigen need to up-regulate in order for an effector T-cell to recognize the antigen? (2)
B7 and CD40
What is the Germinal Center
Part of a follicle where B cells are undergoing high activation
What causes B7 to be expressed on B-cells?
accessory signal molecules IgA and IgB upon activation of the BCR receptor by an antigen
What happens after a T-cell binds with BCR cell (via MHC class II and CD40)? (3)
1) B cell proliferation 2) Initial antibody production 3) germinal center reaction
Characteristics of primary response to an infection? Lag after immunization? Peak response? Antibody isotype? Antibody affinity?
Lag after Immunization: 5-10 days Peak response: small antibody isotype: usually IgM more than IgG antibody affinity: lower average affinity
Characteristics of secondary response to an infection? Lag after immunization? Peak response? Antibody isotype? Antibody affinity?
Lag after immunization: 1-3 days peak response: larger antibody istoype: increase in IgG and other antibodies depending on situation Antibody affinity: higher average affinity
What happens to antibodies during B-cell differentiation in a secondary exposure? (3)
1) Affinity maturation 2) Switch from membrane to secreted form of antibodies 3) Isotype switching
What is Neutralization in regards to antibody action? (4)
antibodies bind to a pathogen and… 1) prevent entry 2) Prevent microb/antigen binding to host cells 3) block spread of infections 4) block binding of toxins to receptors on host cells
What are the isotypes of antibodies? What type of constant chain are each? (5)
IgA: alpha IgD: gamma IgE: E (greek letter) IgG: y (greek letter) IgM: u (mu)
What does IgD do?
Doesn’t really do anything, provides a good marker because it is on all naive B cells
What are J-chain links?
Links the two terminal Fc regions of secreted IgA and IgM.
What is the structure of secreted IgM?
A pentameric structure with it’s constant regions linked by J-chain links. Variable chains on the periphery of the pentamer
What is an Idiotype?
Shared characteristic between IG molecules with shared antigen binding specificity. Variable regions are the same. (ex. IgG for one antigen and IgM for SAME antigen have same Idiotype)
What is an Isotype?
type of antibodies with same constant heavy region (ex. IgG for one antigen and IgG for another antigen have same Isotypes)
How do Ig heavy chains switch?
An intronic region of a gene called a “switch” region
How do antibodies change in T independent activation?
The 3 prime region of the constant region changes so IgM goes from membrane bound to secreted
How do antibodies change in T independent activation? What protein mediates this recombination?
an intronic “Switch” region of the Ig gene that is associated with a constant region switches with a different switch region (that is associated with a different constant region) and that intron is taken out. Results in an antibody with a different constant region. Protein mediating: AID
What T cell and cytokine induces switching to IgG isotype?
Th1 helper T cell releases IFN-y
What T cell and cytokine induces switching to IgE isotype?
Th2 T cell releases IL-4
What cytokine induce switching to IgA isotype?
TGF-beta
How does affinity maturation occur?
proliferating/differentiating B cells accumulate mutations in their variable regions. These B cells undergo apoptosis unless their Ag receptors bind with antigens attached to follicular Dendritic cells, who release a survival signal upon high affinity binding
Where does high affinity maturation occur?
Germinal centers of follicles
What cells help the affinity maturation of B cells? (2)
Follicular dendritic cells and Follicular helper T cells
Where do mutations accumulate in the Ig gene during affinity maturation? (2)
In the hyper-variability regions (CDR1, CDR2, CDR3) of the variable region of heavy chain (Vh) and variable region of light chain (VL)
Why does a booster shot increase immunity?
Because it results in a stronger proliferation, and thus a higher number of somatic mutations during affinity maturation, resulting in higher affinity binding with the antigen
What is the B cell response in T-dependent activation? (3)
Isotype switching, affinity maturation, memory
What is the B-cell response to T-independent activation?
Low level isotype switching of IgM membrane bound to IgM secreted
Which antibodies cause Opsonization and phabocytosis of microbes?
IgG
Which antibodies neutralize microbs and toxins?
IgG and IgA
Which antibodies cause antibody-dependent cellular cytotoxicity?
IgG and IgE
What is opsonization?
process of coating particles for phagocytosis
What is an opsonin?
molecule that coat particles to enhance phagocytosis
How do opsonins bind to phagocytes?
The Fc portion of the Ig bind to a Fc receptor (FcR) on the phagocyte
What occurs after FcR activation? (2)
1) phagocytosis of the antibody coated microbe 2) activates phagocytes to increase degradation of substance via oxidative burst
What is the major host defense mechanism for encapsulated bacteria?
opsoniztion and phagocytosis
Where is the major site of phagocytosis of opsonized bacteria?
spleen
What is the threshold for activation of FcR signaling?
cross-linking of multiple IgG’s to the FcR
What types of Fc Receptors are there and what are their affinities for the Fc region? (4)
1) FcyRI (high affinity) 2) FcyRIIB (low affinity) 3) FcyRIIIA (low affinity) 4) FceRI (high affinity)
What cells are killed via antibody-dependent cellular cytotoxicity? What cells do the killing? (2)
NK cells kill Cancer cells Eosinophils kill Helminths
What Fc receptor do NK cells possess?
FcyRIIIA
What type of Fc receptor is expressed on Eosinophils?
FceRI
What is the cascade for the Alternative complement pathway?
1) C3 is spontaneously hydrolyzed in the serum to C3b, which is stabilized in the presence of microbe 2) Factor B binds C3b and is cleaved to Bb 3) Bb binds to C3b: C3bBb = Alternative C3-convertase 4) Alternative C3-convertase cleaves C3 into C3b and C3a 5) the C3b binds to C3 convertase to become C5 convertase 6) C5-convertase cleaves C5 to C5b and C5a
What is the cascade for the Classical compliment pathway?
1) IgM and IgG bind to microbial surface - C1 binds to two Fc regions of Ig 2) C1 cleaves C4 and C2 into C4b + C2a (classical C3-convertase) 3) Classical C3 convertase cleaves C3 into C3b + C3a 4) C3b binds to C3 convertase to form C5 convertase 5) C5 convertase cleaves C5 into C5b + C5a
What is Alternative C3-Convertase?
C3bBb
What is Classical C3-convertase?
C4b2a
What is the cascade for the Lectin compliment pathway?
1) Mannose-binding lectin (MBL) binds to mannose on a microbe 2) This complex cleaves C4 and C2 into C4b + C2a (classical C3-convertase) 3) Classical C3 convertase cleaves C3 into C3b + C3a 4) C3b binds to C3 convertase to form C5 convertase 5) C5 convertase cleaves C5 into C5b + C5a
What does C3b do?
opsonin and convertase
What does C3a do?
inflammation: bind to PMNs and cause inflammation and PMN oxidative burst
What does C4 do?
protease
What does C4b do?
opsonin and convertase
What does C4a do?
inflammation: binds to PMN’s and causes inflammation and oxidative burst
What does C5a do?
inflammation: binds to PMN’s and cause inflammation and oxidative burst
What does C5b do?
Is the molecule that initiates creation of the membrane attack complex (MAC)
What is the Membrane attack complex (MAC)? structure? Function?
Structure: C6, C7, C8 and C9 bind to C5b sequentially function: C9 binds and polymerizes with C5b to form a pore in the microbe membrane and kills it
What compliment proteins contribute to inflammation? (3)
C3a, C4a, C5a
Which compliment proteins cause opsonization and phagocytosis? (2)
C3b and C4b
What MEMBRANE proteins regulate compliment proteins via inhibition? (3)
MCP, DAF and CR1
What is the mechanism and function of MCP?
MCP is a cofactor for Factor I, which cleaves C3b and C4b into inactive proteins Function: prevents formation of C3 convertase
What is the mechanism and function of DAF?
mechanism: inhibits Bb binding to C3b function: Function: prevents formation of C3 convertase
What is the mechanism and function of CR1?
mechanism: cofactor for Factor I to cleave C3b or C4b into inactive forms Function: prevents formation of C3 convertase
What is the mechanism and function of Factor I?
Mechanism: Cleaves C3b and C4b into inactive forms Function: prevents formation of C3 convertase
What is the mechanism and function of C1 inhibitor?
mechanism: Inhibits action of C1 protease Function: Prevents Classical and Leptin pathways
What SOLUBLE proteins regulate compliment proteins via inhibition? (2)
C1 inhibitor, Factor I
Where is IgA’s main site of action?
Outside of the body in the gut mucosa
How is IgA transported outside of the body into the gut mucosa?
IgA’s J chain binds to the poly-Ig-receptor and is transported accross the epithelium
What is the neonatal FcRn? Where expressed? Function?
Where: apical side of gut mucosa of a baby Function: This is a transporter that brings mother’s IgG into the body
What is alum (aluminum hydroxide gel)?
Alum is an adjuvant that up-regulates the B7 co-stimulatory molecule on macrophages so that the T-cell will recognize the bacterial antigen that you want to protect against
