Indirect Immunoassays

The enzyme-linked immunosorbent assay (ELISA), also known as an indirect immunoassay, is a third type of immunoassay commonly used to detect the presence of antibodies to specific antigenic sites on viruses. These assays have been used extensively in HIV and hepatitis testing. This “true” ELISA format uses a solid phase coated with antigen. Samples are then applied to the solid phase. Any antibodies in the sample will bind to the antigen. A second antibody, labeled with an enzyme, is added to the reaction. This antibody is specific to IgG or IgM and binds to the sample antibody, which is bound to the target antigen. The presence of enzyme is detected by addition of a colorimetric substrate.