ELISA
The Enzyme Linked Immuno-Sorbent Assay (ELISA or EIA for short) is a method commonly employed in biochemistry to detect if a certain substance is present in a sample. It employs antibody specific to the substance; these antibody are linked to an enzyme which produces a signal.
The steps of the general ELISA test are as follows:
- Apply the sample to some sticky substrate, usually a plate with wells.
- Apply the enzyme-linked antibody and let it bind to the substance.
- Wash the plate, so that unbound antibody is removed.
- Apply a chemical which is converted by the enzyme into a fluorescent signal.
- View the result: if it fluoresces, then the sample contained the substance.
The enzyme acts as an amplifier: even if only few enzyme-linked antibody are present, the enzyme molecules will produce many fluorescent signal molecules.
A variant of this techniqe is used in medicine to detect if a person's blood contains antibody against a certain antigen (which would indicate past or present infection). The initial screening test for HIV infection is such an ELISA test. The steps are as follows:
- Prepare a plate to which the antigen is bound
- Apply the human serum to be tested
- Wash the plate, so that unbound antibody is removed.
- Apply an enzyme-linked antibody which specifically binds to human antibody.
- Wash the plate, so that the unbound enzyme-linked antibody is removed.
- Apply a chemical which is converted by the enzyme into a fluorescent signal.
- View the result: if it fluoresces, then the serum sample contained antibody against the antigen.
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