IgG molecules are monomers composed of two light chains (kappa or lambda) and two gamma heavy chains. Approximately 80 % of serum immunoglobulin is IgG; its main task lies in defense against microorganisms, direct neutralization of toxins and induction of complement fixation. IgG is the only immunoglobulin that can cross the placental barrier and provide passive immune protection for the fetus and newborn. This protection gradually catabolizes until the infant's own immunological system begins at about six months of age. Near‑adult levels are reached by 18 months.
Polyclonal IgG increases may be present in systemic lupus erythematosis, chronic liver diseases (infectious hepatitis and Laennec’s cirrhosis), infectious diseases and cystic fibrosis. Monoclonal IgG increases in IgG‑myeloma. Decreased synthesis of IgG is found in congenital and acquired immunodeficiency diseases and selective IgG subclass deficiencies, such as Bruton type agammaglobulinemia.
Decreased IgG concentrations are seen in protein‑losing enteropathies, nephrotic syndrome and through the skin from burns. Increased IgG metabolism is found in Wiskott‑Aldrich syndrome, myotonic dystrophy and with anti‑immunoglobulin antibodies.
The determination of IgG in cerebrospinal fluid (CSF) is used for evaluation of infections involving the central nervous system (CNS), neoplasms or primary neurologic diseases (in particular, multiple sclerosis). Increased CSF IgG concentrations may occur because of either increased permeability of the blood‑brain barrier or local/intrathecal production of IgG, or both.
Malfunction of the blood‑brain barrier can be reliably quantified by means of the albumin CSF/serum ratio. An elevated albumin ratio is an indication of a disorder of the blood‑brain barrier. If IgG and albumin are measured in CSF and serum simultaneously, differentiation between IgG originating from blood and IgG originating from intrathecal production is possible.