What are the five antibody isotypes, and how do their functions differ?

Why

The heavy chain constant region sets the isotype, and class switching changes it without changing the antigen specificity. Secreted IgM is usually a pentamer. Each binding site has low affinity, but ten together give high avidity, and IgM activates complement strongly.

It dominates the early response. IgG is about three quarters of serum antibody. It has four subclasses, opsonizes antigen for phagocytes through Fc gamma receptors, can fix complement, and is carried across the placenta.

IgA is mostly a monomer in serum but a dimer in secretions, bound to a J-chain and a secretory component. It neutralizes microbes at mucosal surfaces and is abundant in colostrum. IgE has the lowest serum level and shortest half-life, yet binds its receptor on mast cells and basophils so tightly that tiny amounts trigger allergy and fight parasitic worms.

IgD appears with IgM on mature naive B cells through alternative splicing. For drug developers this matters because a therapeutic antibody's isotype and subclass set its effector function and half-life. IgG4 and IgG2, for example, fix complement poorly compared with IgG1.