Antibodies are Y shaped glycoproteins called immunoglobulins.Different antibodies bind specifically to a different antigen on the pathogen or toxin that has triggered the immune response. There are millions of different types of antibody and there is a specific antibody for each antigen.Antibodies have two long identical polypeptide chains called heavy chains and two short identical polypeptide chains called light chains which are held together by disulfide bridges.Disulfide bridges also contribute to the tertiary structure of the polypeptide chains giving them their 3D shape.Antibody binding sites are complementary to the shape of antigens and bind via a lock and key mechanism similar to the mechanism of enzyme-substrate formation.The binding site is an area of 110 amino acids on both the heavy and light chain. This is known as the variable region.The rest of the antibody molecule is always the same so it is called the constant region.When an antibody binds to an antigen, it forms an antibody-antigen complex.The hinge region of an antibody provides the molecule with flexibility allowing it to bind to two separate antigens, one at each of its antigen-binding sites.