Immunoglobulin monomer, molecular model. Immunoglobulin (Ig) proteins, also known as antibodies, are gamma globulin proteins that are part of the mammalian immune system. The Ig monomer is a Y-shaped molecule that consists of four polypeptide chains: two heavy chains (green) and two light chains (yellow). The antibody function of immunoglobulin arises from variations in the structure of the tips of the arms of the 'Y' (the antigen-binding sites), allowing millions of possible antibodies to defend against a wide range of pathogens.