What is an immunoassay?
Immunoassays are biochemical tests used to detect the presence or concentration of a specific chemical, such as a toxin or hormone, in a solution using antibody-antigen reactions. Antibodies are Y-shaped proteins produced by the immune systems of humans and other animals in response to foreign substances (antigens) like bacteria, toxins and viruses. Antibodies are antigen-specific, with the binding sites on the antibody and antigen acting as a sort of lock and key. Immunoassays use this antibody-antigen bonding relationship to identify specific molecules in a solution. In immunoassays, antigens are called analytes.
What are the different types of immunoassay?
- An enzyme immunoassay (EIA) uses an enzyme linked to an antibody to detect the antibody-antigen compound via enzymatic reactions. EIAs are commonly used because they are safe and simple. Some examples of EIA include pregnancy tests or HIV tests.
- Enzyme-linked immunosorbent assay (ELISA) is another term for EIA. There are four main types of ELISA: direct, indirect, sandwich and competitive. In each type, the basic principle of detection is consistent, but the arrangement of antigen and antibody changes. In a lab, these tests are usually conducted in multiwell plates, and there are several steps that can be followed to maximize ELISA success.
- Radioimmunoassay (RIA) uses radioactive isotopes, often isotopes of iodine attached to tyrosine, to label the antibody-antigen complex. By measuring the radioactivity, one can determine the quantities of the antibody-antigen compound. While once popular, RIA has become less common due to complications that come from working with radioactive substances.
- Fluorescent immunoassay (FIA) uses fluorophores as the label, measuring fluorescence to determine analyte concentration. Background radiation from the sample itself can complicate FIA, so elements called lanthanides are often used. The emissions from such elements have a decay time far longer than that usually produced by biological samples, allowing the FIA signature to be discerned.
- Chemiluminescence immunoassay (CLIA) combines chemiluminescence with immunochemical reactions. CLIAs use enzymes that catalyze chemical reactions that emit photons. Light emission, or luminescence, is measured to yield data on analyte quantities.
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