ABSTRACT
After exposure to an antigen, the first antibodies to appear are IgM. Later, antibodies of other classes appear: IgG predominates in the serum and extravascular space, while IgA is produced in the gut, and IgE may be secreted at other epithelial surfaces. The different effector functions of these different classes, or isotypes, are summarized in Fig. 11.1. The changes in the isotype of antibody produced in the course of an immune response reflect the occurrence of isotype switching in the B cells that synthesize antibody, so that the
Fig. 11.1 Each human immunoglobulin isotype has specialized functions and a unique distribution. The major effector functions of each isotype (+++) are shaded in dark red, while lesser functions (++) are shown in dark pink, and very minor functions (+) in pale pink. The distributions are similarly marked with actual average levels in serum shown in the bottom row. *IgG2 can act as an opsonin in the presence of Fc receptors of a particular allotype, found in about 50% of Caucasians.
