Antibodies are small, soluble Y-shaped proteins made up of four polypeptide chains. They are called immunoglobulins (Ig) and there are various types. The most common is IgG.There are a pair of shorter polypeptides known as light chains and a pair of longer polypeptide chains known as heavy chains.The chains are linked together by disulfide bridges.The light chains are made of about 200 amino acid residues and the heavy chains have about 400 amino acid residues.There is a variable region at the end of each arm which binds to the antigen to form an antibody-antigen complex.The shape of the variable region is complementary to the shape of the antigen.The rest of the antibody is a constant region which can bind to a receptor on cells such as B cell and phagocytes.The hinge region allows the antibody flexibility so that it can bind more than one pathogen.