Enzyme immunoassay. Researcher uses a multichannel pipette in an enzyme-linked immunosorbent assay (ELISA). ELISA tests are used in diagnosis of immune system reactions, such as those caused by viral and bacterial infection. The surface of a plate is prepared with a covering of a specific protein which binds to the antibody or antigen being tested for. The surface is then exposed to the blood and if the antibody or antigen in question is present it will bind to the surface. Its presence is then shown if a third protein with an attached enzyme binds to it and causes a colour change. The computer shows plots of the reaction.