ELISA

The Enzyme Linked Immuno-Sorbent Assay or ELISA 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
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.

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