A discipline concerned both with the structure of antibody (immunoglobulin) molecules and with their ability to bind an apparently limitless number of diverse chemical structures (antigens); with the structure, organization, and rearrangement of the genes coding for the immunoglobulin molecules; and with the structure and function of molecules on the surface of animal cells, such as the transplantation (histocompatability) antigens, which recognize antibodies and the thymus-derived lymphocytes mediating the cellular immune response. A development of immunochemistry that is of medical, scientific, and commercial importance uses the binding specificity of immunoglobulins to measure complex (but sometimes simple) chemical structures such as alkaloids, hormones, proteins, peptides, complex carbohydrates, and lipids. The tests used for such measurements employ the binding of radioactive antigens (the radioimmunoassay) or the ligation of an antigen bound to an enzyme (the enzyme-linked immunoassay). These assay methods are highly sensitive and specific.