Immunochemical procedures provide protein assay techniques of high sensitivity and discrimination. These methods employ antibodies, proteins produced by an animal’s immune system in response to the introduction of a foreign protein and which specifically bind to this foreign protein.
Antibodies extracted from the blood serum of an animal that has been immunized against a particular protein are the products of many different antibody-producing cells. They therefore form a heterogeneous mixture of molecules, which vary in their exact specificities and binding affinities for their target protein. Antibody-producing cells normally die after a few cell divisions, so one of them cannot be cultured (cloned) to produce a single species of antibody in useful quantities. Such monoclonal antibodies may be obtained, however, by fusing a cell producing the desired antibody with a cell of an immune system cancer known as a myeloma. The resulting hybridoma cell has an unlimited capacity to divide and, when raised in cell culture, produces large quantities of the monoclonal antibody.
A protein can be directly detected through its precipitation by its corresponding antibodies or, in a so-called radioimmunoassay; it can be indirectly detected by determining the degree with which it competes with a radioactively labeled standard for binding to the antibody.
Both radioimmunoassay and ELISAs are widely used to detect small amounts of specific proteins and other biological substances in both laboratory and clinical applications. For example, a commonly available pregnancy test, which is reliably positive within a few days post-conception, uses an ELISA to detect the placental hormones chorionic gonadotropin in the mother’s urine.