Immunoglubulin G (IgG) is a glycoprotein molecule which belongs to the immunoglobulin family of proteins known as antibodies. Immunoglobulins are the key component of humoral immunity. IgG has an approximate molecular weight of about 150kDa and it is composed of four peptide chains: two identical heavy chains (γ) of about 50kDa and two identical light chains (κ) of about 25kDa each. The heavy chains are linked to each other and to the light chain by disulfide bonds. At the N terminus, both the heavy and the light chain contain variable regions (VH and VL) which account for antibody diversity. At the C terminus, both chains contain constant regions (CH and CL) but only CH mediates effector functions. Structurally the IgG molecule may be divided into: (1) the Fragment antigen binding region (Fab) containing the VL, VH, CL and CH2 all of which shape the antigen binding site and (2) the Fragment crystallizable region (Fc) containing CH domains 2 – 4 which stabilize the antibody and bind to the Fc receptor on the surface of macrophages, neutrophils, natural killer cells as well as to complement proteins to mediate therefore physiological effects.

IgG is synthesized and secreted by plasma B cells in response to an immunogen after recognition of specific epitopes on the antigen and it is generated following class switching and maturation of an antibody response, thus providing immune protection. There are four subclasses of IgG in Humans (IgG 1, 2, 3, 4) with variable affinity to Fc receptors and complement. It has been shown that during HIV-1 infection, antibodies with the same epitope specificity but of different subclass have different functions.

IgG3 comprises around 5 - 10% of total IgG and plays a major role in the immune responses against protein or polypeptide antigens. The affinity of the Fc region of IgG3 binds with very high affinity and mediates a strong antibody-dependent cellular cytotoxicity and complement dependent cytotoxicity. IgG3 does not bind to Protein A, but it does bind to Protein G and L. Antigen-specific IgG3 immune response has been associated with long-term protection from the Plasmodium falciparum parasite and Chikungunya virus and it has been shown to be correlated with a decreased HIV infection risk after vaccination induced immediate humoral response.

IgG3 deficiency is associated with Wiscott-Aldrich disease, systemic lupus erythematosus, juvenile diabetes mellitus, and peridontal infections. IgG3 deficiency has been associated with a history of recurrent infections, leading to chronic lung disease

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