proteins also known as antibodies made by the B cells of the immune system in response to a specific antigen, and containing a region that binds to this antigen (antigen-binding site). Each immunoglobulin is made up of two polypeptides, a heavy chain and a light chain. These heavy and light chains complex to form the antigen binding site. Each group of immunoglobulins is named for the type of heavy chain that it contains. The heavy chain determines the post-antigen binding activity of an antibody. There are five classes of immunoglobulins: IgA, IgD, IgE, IgG, and IgM.