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Immunoglobulin G (IgG) is a type of antibody. 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. IgG molecules are created and released by plasma B cells. Representing approximately 75% of serum antibodies in humans, IgG is the most common type of antibody found in the circulation. By binding many kinds of pathogens such as viruses, bacteria, and fungi, IgG protects the body from infection. IgG also plays an important role in antibody-dependent cell-mediated cytotoxicity (ADCC) and intracellular antibody-mediated proteolysis, in which it binds to TRIM21) in order to direct marked virions to the proteasome in the cytosol. IgG is also associated with type II and type III hypersensitivity reactions.