Immunoglobulin pentamer, 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 (yellow) and two light chains (green). This form of Ig is IgM, five monomers of which have joined together to form a pentamer. An extra chain, the J chain (red), helps bind the pentamer together. Pentameric IgM has 10 binding sites compared to the normal two of a monomer.