ELISA, Enzyme Linked Immunosorbent Assay is an immunochemical measurement method where antibodies or antigens that are measured have been labeled with enzymes. The presence of antigen or antibody is detected by the ability of the enzymes to e.g. induce a color change in the test.
The assay format combine the specificity of antibodies with the sensitivity of simple enzyme assays, by using antibodies or antigens coupled to an easily-assayed enzyme. ELISAs can provide a useful measurement of antigen or antibody concentration.