An antibody is a type of immune molecule that is produced when a virus, or antigen, invades the body. Each antigen typically produces a specific type of antibody that can be detected using an assay test in a lab. The antibody usually has a certain affinity for an antigen, which can indicate the strength of the reaction that could occur. Antibodies also have a measurable avidity, which is the strength of the binding that occurs with the antigen when they are combined.
Enzyme-linked immunosorbent assay (ELISA) is a biochemical test used to detect the presence of an antigen in a sample of blood, which can be identified by its affinity and avidity. During the test, a sample of an unknown antigen usually is affixed to the surface of a test plate. A specific antibody, known as the ELISA antibody, is washed over the surface so that it can bind to a receptor site on the antigen. Using a reagent to produce a change of color in the solution usually helps identify the type and amount of antigen in the sample.
The ELISA antibody used in the test can be monoclonal, meaning that it only will detect a single known antigen. Polyclonal antibodies can be used for identifying multiple antigens. Typically, polyclonals are less expensive to use, but monoclonals are preferred when specific results are desired.
ELISA is the most commonly used immunological assay test because it is able to detect small amounts of an antigen. It also has the ability to distinguish between two closely related molecules. Known as a rapid-acting test, it quickly can give results.
The ELISA antibody test has been used to evaluate the presence of antibodies in human immunodeficiency virus (HIV) tests. It also has been used during testing for the West Nile Virus and for Hepatitis C. The food industry uses the test to detect allergens such as milk, peanuts, and eggs in consumer products.
False-positive results of an infection can come from an ELISA antibody test, however. For example, a patient that tests positive for Hepatitis C antibodies does not necessarily have an active hepatitis infection. The patient could have been infected in the past, but the body continues to produce the antibodies. Other causes of false positives include the presence of an autoimmune disorder or an impure reagent used during the assay.