The Enzyme Linked Immunosorbent Assay (ELISA) is a test that uses antibodies and color change to identify a substance. The ELISA has been used as a diagnostic tool in medicine and plant pathology, as well as a quality-control check in various industries. An ELISA test uses components of the immune system and chemicals to detect immune responses in the body (for example, to infectious microbes). The ELISA test involves an enzyme (a protein that catalyzes a biochemical reaction). It also involves an antibody or antigen (immunologic molecules). The purpose of an ELISA is to determine if a particular protein is present in a sample and if so, how much. Because the ELISA can be performed to evaluate either the presence of antigen or the presence of antibody in a sample, it is a useful tool for determining serum antibody concentrations. ELISA tests are generally accurate tests. They are considered highly sensitive and specific and compare favourably with other methods used to detect substances in the body, such as Radioimmune Assay (RIA) tests. ELISA can also be used in toxicology as a rapid presumptive screen for certain classes of drugs.