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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:

1. Apply the sample to some sticky substrate, usually a plate with wells.
2. Apply the enzyme-linked antibody and let it bind to the substance.
3. Wash the plate, so that unbound antibody is removed.
4. Apply a chemical which is converted by the enzyme into a fluorescent signal.
5. 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:

1. Prepare a plate to which the antigen is bound
2. Apply the human serum to be tested
3. Wash the plate, so that unbound antibody is removed.
4. Apply an enzyme-linked antibody which specifically binds to human antibody.
5. Wash the plate, so that the unbound enzyme-linked antibody is removed.
6. Apply a chemical which is converted by the enzyme into a fluorescent signal.
7. View the result: if it fluoresces, then the serum sample contained antibody against the antigen.

External links:

• Online bookshelf from NCBI: searchable text books in molecular biology and related fields, http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=Books