Y

Monomer

IgG, IgE, IgD

Dimer

IgA

Secretory

■“ component

Pentamer

IgM

—J chain

C H A P T E R 3 5 , F IG U R E 11

Immunoglobulin classes. The five immunoglobulin classes are determined by the

amino acid sequences of the constant regions of the heavy polypeptide chains (see Table 35-2).

Equivalence of antigen and antibody

Excess antigen

Excess antibody

C H A P T E R 3 5 , F IG U R E 1 2

Antigen-antibody complex formation. Optimal concentrations of antibody and anti-

gen result in cross-linking and precipitation of the antigen-antibody complexes. Cross-linking is important in B-cell

stimulation when antigens bind to the IgM B-cell receptors. If antibody is present in excess, then all of the epitopes

on the antigen can be occupied by single antibody molecules (subject to steric exclusion) and the antigen-antibody

complexes do not precipitate. If antigen is present in excess, then all antibody binding sites can be occupied by anti-

gen molecules and no cross-linking occurs.