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What Is an Enzyme-Linked Immunosorbent Assay?

Jennifer Long
Jennifer Long

An enzyme-linked immunosorbent assay (ELISA) is a type of test that is used to measure antibodies or antigens in a fluid sample. Also known as an enzyme immunoassay (EIA) test, an ELISA is a biochemical technique that is commonly used in immunology. It is used to diagnose conditions that may trigger an immune system response. Enzymes are used to tag antigens or antibodies to determine the type of disease, illness, or allergy a person has.

Blood tests are beneficial aids in diagnosing many different problems. Diseases such as cancer and autoimmune conditions can be detected with an enzyme-linked immunosorbent assay. An ELISA test can be used to target specific antibodies that are made by the immune system to fight foreign invasions. ELISA tests are also used to determine specific allergies a person may have.


Enzymes that are used for an enzyme-linked immunosorbent assay are specific proteins that cause a reaction with antibodies or antigens that are targeted. A blood sample is taken and added to a test plate that has multiple wells line up in columns. In these wells, a solution that contains a condition-specific antigen is first, with the blood sample layered on top. A primary antibody is added to the two fluids, which binds to the antigens. Secondary antigens are added that bind to the primary antibodies, followed by the enzyme.

Once the enzyme has been added, the enzyme-linked immunosorbent assay will begin to change colors if the sample contains a specific antigen or antibody associated with a condition. Initial color changes show that primary and secondary antibodies have bound together. When a color change has occurred, the results are considered positive. A positive result shows the presence of a condition. In an ELISA test, it is common for multiple antigens to be used to rule out several conditions or allergies at once.

There are varying degrees of color change with positive results. An enzyme-linked immunosorbent assay can have brighter or lighter coloring depending on the concentration of antibodies or antigens in a sample. For example, if the antibodies of a condition are higher than usual, the color change will be much brighter. With many conditions, these results can aid doctors in determining the severity.

Enzyme-linked immunosorbent assay tests are sensitive tests and generally accurate. Although the ELISA test was first used to test for the human immunodeficiency virus (HIV), it has many applications today. Generally, the ELISA test is used in the medical field, but it can also be used in plant biology testing and industrial quality testing.

Radioimmunoassay was a similar test that was used prior to the development of the ELISA test. This version uses radioactive antigens and antibodies to test samples instead of enzymes. While some laboratories still use this technique, most will use the ELISA test because it is safer to perform than its predecessor.

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