Antibodies are composed of two identical heavy chains and two
light chains. Initially, all mature B cells express IgM and IgD
on their cell surface. However, upon exposure to antigen, and under
the influence of cytokines and stimulatory surface interactions
with T cells, B cells can undergo isotype switch differentiation.
By this process, B cells bearing surface IgG, IgA, and IgE surface
receptors are generated, and antigen receptor specificity is preserved.
IgM, the first immunoglobulin (Ig) to be generated in a primary
immune response, is secreted as a pentamer. IgG is the most abundant
Ig in human serum, composing 80% of the total Ig. IgG is
divided into four subclasses, based on structural differences. IgA
comprises 10% to 15% of serum Ig and contains two
subclasses, IgA1 and IgA2. Secreted IgA2 is the major Ig at mucosal
surfaces, where it exists as a complex comprising two IgA monomers,
a joining chain, and a secretory component. IgE plays a role in
immunity to parasites and mediates allergic reactions.