Clinical Significance

IgG is the major immunoglobulin in the blood and is produced in copious amounts during secondary immune responses.  IgG molecules bind to specific receptors on phagocytic cells, such as macrophages and polymorphonuclear leukocytes, thereby increasing the efficiency with which the phagocytic cells can ingest and destroy infecting microorganisms that have become coated with IgG antibodies in response to the infection.  IgG molecules are the only antibodies that can pass from mother to fetus.  The ability of IgG to cross the placenta provides a major line of defense against infection for the first weeks of an infant’s life.  Quantitation of IgG can be used to evaluate humoral immunity, establish diagnosis and monitor therapy in IgG myeloma, and evaluate patients, especially children and those with lymphoma, with propensity to infections.  Reduction of IgG, usually less than 300mg/dL, leads to susceptibility to infection due to encapsulated bacteria.  Elevated IgG levels can be polyclonal, oligoclonal, or monoclonal.
Elevated polyclonal IgG levels are associated with autoimmune diseases (systemic lupus erythematosus, rheumatoid arthritis, and Sjogren’s syndrome), chronic liver disease, some parasitic diseases, chronic or recurrent infections, and intrauterine contraceptive devices.  Increased oligoclonal IgG levels are associated with malignancies, infections (especially in the elderly), some dysgammaglobulinemias, and autoimmune disorders.  Increased monoclonal IgG levels are associated with multiple myeloma (IgG type), lymphomas, and leukemia.