immunoglobulin complex IgG immunoglobulin complex IgG immunoglobulin complex A protein complex composed of two identical immunoglobulin heavy chains of an IgG isotype and two identical immunoglobulin light chains, held together by disulfide bonds. An IgG immunoglobulin complex may be embedded in the plasma membrane or present in the extracellular space, in mucosal areas or other tissues, or circulating in the blood or lymph.