IgGs

IgGs, or immunoglobulin G antibodies, are the most abundant class of antibodies in human serum. Each IgG molecule is a monomer composed of two identical heavy chains (gamma chains) and two light chains (kappa or lambda), forming Fab regions that bind antigen and an Fc region that engages immune effector mechanisms.

There are four IgG subclasses (IgG1–IgG4). They differ in hinge length, Fc receptor affinity, and ability to activate complement. IgG1 and IgG3 are most effective at Fc receptor engagement and classical complement activation; IgG2 responds well to polysaccharide antigens but less to protein antigens; IgG4 is functionally anti-inflammatory, has a tendency for Fab-arm exchange, and activates complement poorly.

IgG antibodies neutralize pathogens, block toxin activity, and opsonize microbes to promote phagocytosis. They bridge innate and adaptive immunity by binding Fc gamma receptors on macrophages, neutrophils, and NK cells, and can activate the classical complement pathway via C1q binding. IgG subclasses influence these effector functions and their tissue distribution. IgG can cross the placenta, providing passive immunity to the fetus and newborn, mainly through FcRn-mediated transport.

IgG is produced by plasma cells in secondary lymphoid tissues after class switch recombination from IgM/IgD during germinal center reactions, followed by affinity maturation through somatic hypermutation. In adults, serum IgG is the largest component of total Ig and has a half-life of about three weeks due to FcRn recycling.

Clinically, IgG measurements assess humoral immunity and diagnose immunodeficiencies or prior infections. Therapeutically, intravenous immunoglobulin (IVIG) preparations consist of pooled IgG and are used to treat antibody deficiencies and various autoimmune conditions. IgG subclass deficiencies and autoantibody-mediated diseases are areas of active clinical study.