IgG3

IgG3 is one of the four human subclasses of immunoglobulin G (IgG). It is produced as part of the adaptive immune response and contributes to the pool of circulating IgG, typically representing a minority of total IgG with levels that can vary among individuals.

IgG3 has the longest hinge region of the human IgG subclasses, consisting of roughly 62 to 67 amino acids. This long hinge confers notable molecular flexibility, enabling effective cross-linking of antigens with diverse epitopes. The extended hinge also makes IgG3 more susceptible to proteolysis compared with other IgG subclasses and contributes to its distinct pharmacokinetic properties.

In terms of effector functions, IgG3 binds strongly to Fc gamma receptors (FcγRs) on phagocytes and NK cells, promoting antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis. It is also highly efficient at activating the classical complement pathway through C1q binding, which enhances inflammation and pathogen clearance.

Pharmacokinetically, IgG3 has the shortest serum half-life among human IgG subclasses, roughly around seven days, due in part to its interaction with the neonatal Fc receptor (FcRn). It can cross the placenta to aid fetal immunity, but its transplacental transfer is generally considered less efficient than that of IgG1.

Clinical relevance includes monitoring IgG subclass levels to evaluate humoral immune function. Deficiencies or imbalances involving IgG3 can be associated with increased susceptibility to certain infections, particularly respiratory pathogens, and may inform diagnosis and management of immunodeficiency or autoimmune conditions.