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The enzyme linked immunosorbent assay (ELISA) is an immunological technique based on the principle of antigen and antibodies conjugation. In ELISA, antigens from the sample are fixed on the surface of ELISA plate. Then primary antibody is (specific to antigen) added which binds to the antigen. After that an enzyme linked secondary antibodies are applied. Finally the substrate for the enzyme is added. The reaction of the enzyme and substrate gives detectable changes like color change or fluorescence.

ELISA can be used to detect the presence or absence of antigen such as drug allergens, pregnancy hormones or antibodies in a sample such as lyme disease, trichinosis. Even the concentrations of particular antibodies can also be determined. It is the most basic test to determine if an individual is positive for a selected pathogen, such as HIV, common cold. The tool is also useful in toxicology for screening of certain kind of drugs.

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