IgG

IgG constitutes approximately 75-80% of total serum antibody and is the predominant antibody in secondary (memory) immune responses, reflecting class switching from IgM after initial exposure to antigen. Structurally, it is a monomer composed of two heavy chains and two light chains, with four subclasses (IgG1-4) differing in hinge region flexibility and effector functions. IgG functions include: (1) opsonization of pathogens for phagocytosis via Fc receptor binding on macrophages/neutrophils, (2) activation of the classical complement pathway, (3) neutralization of toxins and viruses, and (4) antibody-dependent cellular cytotoxicity (ADCC) via NK cells. Clinically, IgG is crucial in passive immunity transfer from mother to fetus via FcRn-mediated placental transport, providing protection during the first few months of life. Elevated IgG levels can be seen in chronic infections, autoimmune diseases (e.g., SLE, rheumatoid arthritis with rheumatoid factor often IgM but anti-CCP is IgG), and multiple myeloma (monoclonal IgG spike on serum protein electrophoresis). Deficiencies in IgG subclasses or total IgG (as in common variable immunodeficiency, CVID) predispose to recurrent sinopulmonary infections with encapsulated organisms. IgG is also the primary antibody measured in vaccination titers to assess immune memory (e.g., Hepatitis B surface antibody, MMR titers). In autoimmune hemolytic anemia, warm antibody type is typically IgG-mediated (extravascular hemolysis via splenic macrophages), whereas cold agglutinin disease is IgM-mediated.