The exquisite sensitivity and specificity of antigen-antibody reactions has been utilized as the basis for many diagnostic procedures. Depending on the test design, one can detect or assay either specific antigens or specific antibodies. Because serum is most often used in these assays, they are also known by the generic designation of immunoserological tests.
Immunoserological assays can be developed for antigen or antibody detection or quantita-tive assay. For antibody detection or measurement, a purified preparation of antigen must be available. For instance, when human serum is tested for antibody to diphtheria toxoid, a purified preparation of the toxoid must be available. Second, a method for detecting the specific antigen-antibody reaction must be developed. Conversely, to detect antigens in bi-ological fluids, specific antibody must be available. In both types of assays positive and negative controls are required for proper interpretation of the results.
Antigen-detection tests have been applied to the diagnosis of infectious diseases, the detection of previous infection, the monitoring of neoplasms and vaccine efficacy, the as-say of hormones and drugs, and pregnancy diagnosis. Measurement of specific antibodies has found wide application in the diagnosis of infectious, allergic, autoimmune, and im-munodeficiency diseases. A variety of different techniques have been developed over the years for these purposes. Some of the assays for antigen/antibody detection described be- low, such as the diffusion-based and electrophoresis-based assays, are no longer commonly used. These procedures have, for the most part, been replaced by commercial assay kits, which are faster, easier to perform and more quantitative than the original assays. Because the scientific principles of the original assays and the newer technologies are the same, the description of these “historical” methods is included herein.