Antigen-antibody assay. Wells being filled with fluid samples by pipette for an immunoassay test. This is a dilution series, with wells on the left less filled than those at right. An immunoassay (antigen-antibody assay) determines the presence or absence in the blood of a specific protein such as an antigen, an antibody, or a protein like a hormone. For any specific antigen (foreign body) there is an antibody produced by the immune system which will bind to it, and may show up positively in a colour reaction test. Thus, an ELISA test (enzyme-linked immunosorbent assay) is an immuno- assay used to detect antibodies from viral infection such as the HIV virus which causes AIDS.