Immunoglobulin G (IgG) is an antibody isotype. It is a protein complex composed of four peptide chains-two identical heavy chains and two identical light chains arranged in a Y-shape typical of antibody monomers. Each IgG has two antigen binding sites. Representing approximately 75% ofserum immunoglobulins in humans, IgG is the most abundant antibody isotype found in thecirculation. IgG molecules are synthesized and secreted by plasma B cells.