Immunoglobulin (Ig) G (IgG) is the predominant of five classes of Ig (IgG, IgA, IgM, IgE, and IgD). Igs differ in heavy chain structure and effector function. IgG1, the largest IgG subclass, represents ~ 60% of IgG and has a half-life of 21 days. Antibody responses to soluble protein and membrane antigens primarily induce IgG1, although polysaccharides and allergens also elicit IgG1 responses.