The Enzyme-Linked ImmunoSorbent Assay (ELISA) is a biochemical technique used mainly in immunology to detect the presence of an antibody or an antigen in a sample using two antibodies. One antibody is specific to the antigen and the other reacts to antigen-antibody complexes, and is coupled to an enzyme. This second antibody, which accounts for "enzyme-linked" in the assay's name, can also cause a chromogenic or fluorogenic substrate to produce a signal.

The enzyme acts as an amplifier; even if only few enzyme-linked antibodies remain bound, the enzyme molecules will produce many signal molecules.

Because the ELISA can be performed to evaluate either the presence of antigen or the presence of antibody in a sample, it is a useful tool both for determining serum antibody concentrations (such as with the Human Immunodeficiency Virus or West Nile Virus) and also for detecting the presence of antigen. It has also found applications in the food industry in detecting potential food allergens such as milk, peanuts, walnuts, almonds, and eggs.

ELISA may be run in a qualitative or quantitative format. Qualitative results provide a simple positive or negative result for a sample. The cutoff between positive and negative is determined by the analyst and may be statistical. Two or three times the standard deviation is often used to distinguish positive and negative samples. In quantitative ELISA, the optical density or fluorescent units of the sample is interpolated into a standard curve, which is typically a serial dilution of the target.

ELISA, according to the order or combination of using antibody and antigen, can be defined in the following types:

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