IgG

Immunoglobulin G (IgG) is the most abundant class of antibody in human serum and extracellular fluid, playing a central role in the body's defense against bacteria and viruses. Each IgG molecule consists of two identical gamma heavy chains and two light chains (kappa or lambda), forming a Y-shaped structure. The Fab regions at the tips bind specific antigens, while the Fc region mediates interactions with Fc receptors on immune cells and with the complement system. IgG can activate the classical complement pathway via C1q binding and can promote opsonization and phagocytosis. It can also mediate antibody-dependent cellular cytotoxicity (ADCC) through Fc gamma receptors on natural killer cells and macrophages. The Fc region is glycosylated, and its glycan profile influences effector functions.

Humans express four IgG subclasses: IgG1, IgG2, IgG3, and IgG4. IgG1 is the most abundant and efficient at Fc receptor binding and complement activation; IgG2 mainly targets polysaccharide antigens; IgG3 has strong effector functions but a shorter half-life; IgG4 is less inflammatory and can undergo Fab-arm exchange, contributing to anti-inflammatory responses. IgG is transported from mother to fetus across the placenta by the neonatal Fc receptor (FcRn), providing passive immunity to the newborn.

IgG constitutes a significant portion of serum immunoglobulins, with a half-life of about 21 days, and is produced by plasma cells derived from B cells after antigen exposure. Clinically, IgG levels are measured to assess immune status; therapies include intravenous immunoglobulin (IVIG) preparations and monoclonal IgG antibodies. The gamma heavy chain gene locus on chromosome 14 underpins class switching to IgG during B cell maturation.