23). The clinical laboratory appears to be one of the first entities to utilize monoclonal antibodies.The ELISA is used for screening everything from drugs to the AIDS virus in humans.There are a variety of immunoassays which make use of monoclonal antibodies and many could be used in their present form for FDA regulatory work.An example is the two-site immunoassay developed for Listeria spp.It employs two different monoclonal antibodies that recognize two distinct epitopes on the same antigen.The antibodies do not compete with one another.Pure antigen is not required for their formation,and the immunoassay is sensitive and specific.In the double-antibody two-site immunoassay a specific monoclonal antibody directed against the antigen of interestis passively adsorbed ontoasolid phase support (polystyrene).The solid phase is then washed to remove unadsorbed antibody.The antigen in question becomes attached to the first monoclonal antibody.The enzyme-labeled antibody is added,which binds to a different epitope on the already bound antigen.Following incubation, the excess labeled monoclonal antibody is removed by washing.Substrate is added and the absorbance is measured spectrophotometrically.The ELISA may be more sensitive than RIA.In the latter,when the radionuclide emits gamma rays or beta particles,it is less active.In ELISA,the enzyme catalyzes a substrate molecule,but the enzyme can be reused. Purifying Antibodies 23). The clinical laboratory appears to be one of the first entities to utilize monoclonal antibodies. The ELISA is used for screening everything from drugs to the AIDS virus in humans. There are a variety of immunoassays which make use of monoclonal antibodies and many could be used in their present form for FDA regulatory work. An example is the two-site immunoassay developed for Listeria spp. It employs two different monoclonal antibodies that recognize two distinct epitopes on the same antigen. The antibodies do not compete with one another. Pure antigen is not required for their formation, and the immunoassay is sensitive and specific. In the double-antibody two-site immunoassay a specific monoclonal antibody directed against the antigen of interest is passively adsorbed onto a solid phase support (polystyrene). The solid phase is then washed to remove unadsorbed antibody. The antigen in question becomes attached to the first monoclonal antibody. The enzyme-labeled antibody is added, which binds to a different epitope on the already bound antigen. Following incubation, the excess labeled monoclonal antibody is removed by washing. Substrate is added and the absorbance is measured spectrophotometrically. The ELISA may be more sensitive than RIA. In the latter, when the radionuclide emits gamma rays or beta particles, it is less active. In ELISA, the enzyme catalyzes a substrate molecule, but the enzyme can be reused. Purifying Antibodies