Immunoglobulin G is the most abundant antibody isotype found in the circulation. Human IgG4, one of four subclasses of IgG, contains a gamma 4 heavy chain and a hinge region that is shorter than that of IgG1. Its two

primary effector functions are activating complements and binding to the FcgR of effector cells to initiate phagocytosis. Human IgG4 accounts for less than 6% of the total IgG serum level.