Immunodiagnostics is a diagnostic methodology that uses an antigen-antibody reaction as their primary means of detection. The concept of using immunology as a diagnostic tool was introduced in 1960 as a test for serum insulin. A second test was developed in 1970 as a test for thyroxine in the 1970s.
It is well-suited for the detection of even the smallest of amounts of (bio)chemical substances. Antibodies specific for a desired antigen can be conjugated with a radiolabel, fluorescent label, or color-forming enzyme and are used as a "probe" to detect it. Well known applications include pregnancy tests, immunoblotting, ELISA and immunohistochemical staining of microscope slides. The speed, accuracy and simplicity of such tests has led to the development of rapid techniques for the diagnosis of disease, microbes and even illegal drugs in vivo (of course tests conducted in a closed environment have a higher degree of accuracy). Such testing is also used to distinguish compatible blood types.
The Enzyme-Linked ImmunoSorbent Assay or ELISA and the Lateral-Flow test, also known as the dipstick or rapid test, currently are the two predominant formats in immunodiagnostics.
The ELISA (sometimes also called an EIA) is a sensitive, inexpensive assay technique involving the use of antibodies coupled with indicators (e.g. enzymes linked to dyes) to detect the presence of specific substances, such as enzymes, viruses, or bacteria. While there are several different types, basically ELISAs are created by coating a suitable plastic (the solid phase) with an antibody. To complete the reaction, a sample believed to contain the antigen of interest is added to the solid phase. Then a second antibody coupled with an enzyme is used followed by the addition of a color-forming substrate specific to the antibody.