Immunoassays are biomedical diagnostic tests that help to detect the presence of macromolecules such as proteins, hormones, etc. (and even measure their quantity) in a sample of biochemical interest by making use of the specific antibody-antigen binding reaction. The antigen(s) to be quantified are called analyte(s). Best immunoassays are those that are highly sensitive to their specific analytes. Immunoassays can be used to detect infectious and autoimmune diseases. Immunoassays can be of the following types, depending upon the reagents used:
Enzyme Immunoassays (EIA): These immunoassays make use of certain enzymes. They are also called as Enzyme-lined immunosorbent assays, popularly called as ELISA.
Radioimmunoassay (RIA): These immunoassays use radioactive isotopes.
Fluoroimmunoassay (FIA): These immunoassays use fluorescent materials.
Chemiluminescent immunoassay (CLIA): These immunoassays use chemiluminescent materials.
Counting Immunoassay (CIA): These immunoassays employ particle-counting technique.
The reagents can be labeled as well as unlabelled. In case of the labelled ones, immunoassays can be classified as homogenous and heterogeneous. Heterogeneous immunoassays can be classified as "competitive" (when the analyte has to compete with the labeled antigen for binding with the antibody in the immunoassay) and "non-competitive" immunoassays (when the analye first gets bound to the antibody to which the labeled antigen gets bound afterwards).