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ELISA: An important Diagnostic Tool

written by: julienranger•edited by: Emma Lloyd•updated: 2/28/2010

ELISA is a useful diagnostic tool in the field of immunology and serology. Its high specificity and sensitivity has made it indispensable in laboratory diagnosis of diseases.

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    ELISA stands for enzyme-linked immunosorbent assay, also sometimes called EIA or enzyme immunoassay. It is a biochemical technique widely used in the field of immunology and serology. Its primary purpose is to detect whether an antibody or an antigen is present in a sample, and involves the interaction between an antigen and its specific antibody. An enzyme is used to detect whether this interaction took place, either through a change in color, or production of fluorescence.

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    How It Works

    Antigens are molecules or parts of molecules which are able to elicit an immune response from our body's immune system. They are usually present in bacteria and viruses, but are sometimes present in the body's cell in certain pathologic states. Since these bacteria and viruses are considered "foreign", the body's response is to try to eliminate them through antibody production. Antibodies are proteins produced by the body's immune cells in response to these antigens. These antibodies are specific, and will only bind to the antigen which stimulated their production. The antibody will bind to the offending antigen, and this allows the other immune cells in the body to easily detect them since they are "marked" for destruction by these antibodies. ELISA may be classified as direct or indirect, depending on whether the test detects an antigen or antibody.

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    Direct ELISA

    A direct ELISA method detects the presence of an antigen in a particular sample. Antibodies to the antigen about to be detected are affixed to the surface of a plate containing small wells (called a "microtiter plate) for holding the sample, which is usually serum. When the sample is placed on these wells, the antigens in the sample will bind with the antibodies, forming antigen-antibody complexes. An enzyme is "linked" to a second antibody contained in a different solution. This solution is added to the wells, resulting in binding of the second antibody with the formed complexes. This leads to the activation of the enzyme, producing a change in color, or in some cases, development of fluorescence.

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    Indirect ELISA

    In contrast, an indirect ELISA method detects the presence of an antibody in a sample, usually serum. This time, antigens are bound to the surface of the wells, and complexes are formed when antibodies are present in the sample. A second antibody is added and binds to the complex, producing a change in color, or fluorescence.

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    The ELISA test is commonly used to diagnose infectious diseases, whether they are caused by viruses or bacteria. Pathogenic bacteria or viruses (those which can cause disease) usually have antigens which could stimulate antibody production by the body's immune cells. One example is human immunodeficiency virus (HIV) screening, where antibodies to the virus are detected in serum samples of subjects. It is also used as a diagnostic tool in plant pathology, and as a quality-control check in some industries.

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    The University of Arizona: Introduction to ELISA Activity

    Davidson College Department of Biology: ELISA (Enzyme-Linked Immunosorbant Assay)