Introduction Purpose to detect the presence of antigens via the use of antibodies that are conjugated with an enyme that converts a colorless substrate (chromogen) into a colored end product this is different than a radioimmunoassay (RIA) which detects a radiolabeled product Example in an HIV screening test, the HIV's p24 capsular antigen is coated onto a mirotiter plate the patient's serum (which contains their antibodies) is added to this microtiter plate as well as the anti-antibody that is labeled with the enzyme (added after washing) the colorless substrate is added (after washing again), and if the reagents bind a color change will occur