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 is the main type of antibody found in blood and extracellular fluid allowing it to control infection of body tissues. IgG molecules are created and released by plasma B cells. IgG is the only isotype that has receptors to facilitate passage through the human placenta, thereby providing protection to the fetus in utero.
IgG antibodies are extracted from donated blood plasma and used as a therapeutic known as intravenous immunoglobulin (IVIG). This is used to treat immune deficiencies, autoimmune disorders, and infections.