Antigen recognition

B-cell antigen regocnition

recognize with ‘B cell receptor’ (BCR)

produce antibodies to protein antigens

t cells help the b cells to

t cell antigen recognition

t cell receptors (bind with peptides displayed by MCH molecules)

BCR complex (on membrane and antibody)

Ig + Ig-alpha + Ig-Beta

TCR complex

TCR + CD3 and zeta chains (only membrane bound)

receptor cross linking

lymphocyte activation requires

variable region and constant region

what is the structure of antigen receptors

antibody

four polypeptides (2 heavy and 2 light chains; Y-shaped structure)

Fab

fragment antigen binding for antigen recognition (comprised of 2 heavy chains and 2 light chains)

Fc

each one contains one variable and 3 or 4 constant regions

Fc

each one contains one variable and 3 or 4 constant regions

two light chains

each contains one variable region and one constant region; light chain class remains fixed for life once made

isotype switching

changing the antibody type from IgM to IgG; it will have the same variable region (maintains target specificity)

complementarity determining regions (CDR)

antigen recognition region in receptors vary between clones; variable regions; highest variablity in CDR3 region

proteolytic cleavage of antibodies

enzymatic digestion:

1) papain digestion: acts on hinge region, produces 2 Fab fragments (retain binding specificity)

2) pepsin digestion: distal to hinge region, produces F (ab’)2 antigen binding fragment

accomplished by Fab and F(ab’)2

antibody-mediated neutralization of microbes and toxins can be

cannot mediate effector functions

Fab fragments cannot bind complement or Fc receptors so they

binding

a reversible noncovalent interaction

dissociation constant (Kd)

affinity of interaction expressed as (high concentration of Ab and Ag required = strength of binding is low); SMALLER Kd = stronger binding

affinity of binding interations

Kd of primary response is higher (lower affinity), Kd of secondary response is lower (higher affinity)

IgG, IgD, IgE

monomers w/ 2 antigen binding sites

secreted IgA

dimer in mucosal secretion w/ 4 antigen binding sites; monomer in serum

secreted IgM

pentamer, 10 antigen binding sites

IgG > IgA/IgM > IgE (GAME)

concentration of antibodies in the serum

function of IgG

opsonization, compliment activation, cell-mediated cytotoxicity

function of IgM

naive B cell antigen receptor, complement activation

function of IgA

mucosal immunity

function of IgE

defense against helminths, immediate hypersensitivity (allergic reactions)

monoclonal antibodies

antibodies derived from a single plasma cell specific for one epitope on a complex antigen

chimeric antibody

the AA sequence of one species in one region and the sequence of a different species in another

TCR characteristics

heterodimer; one variable and one constant region (CDR3 is most variable); TCRs are always membrane-anchored (not secreted, not class switching)

TCR affinity

low affinity, slow binding; binding of T cells to APC has to be strengthened by accessory molecules