Thirty five years ago, the ELISA test was discovered by two Swedish scientists, Eva Engvall and Peter Perlman. The Enzyme-Linked ImmunoSorbent Assay (ELISA for short) is a powerful biochemical tool commonly used in clinical diagnostics today. Using the principle of antigen-antibody interaction, the assay allows detection of proteins, such as those making up cellular components of bacteria and viruses, in solutions such as serum, urine, or culture media. Aside from detecting pathogens, the ELISA has also been adapted to detect food allergies, particularly for the treatment of Irritable Bowel Syndrome.

How does it work? Here's how the general method runs, as described in Wikipedia:

• Apply a sample of known antigen to a surface, often the well of a microtiter plate. The antigen is fixed to the surface to render it immobile.
• The plate wells or other surface are then coated with serum samples of unknown antibody concentration, usually diluted in another species' serum. The use of non-human serum prevents non-specific antibodies in the patient's blood from binding to the antigen.
• The plate is washed, so that unbound antibody is removed. After this wash, only the antibody-antigen complexes remain attached to the well.
• The second antibodies are added to the wells, which will bind to any antigen-antibody complexes. These second antibodies are coupled to the substrate-modifying enzyme.
• Wash the plate, so that excess unbound antibodies are removed.
• Apply a substrate which is converted by the enzyme to elicit a chromogenic or fluorescent signal.
• View/quantify the result using a spectrophotometer or other optical device.

Other versions include the "Sandwich ELISA and the "Competitive ELISA". The University of Arizona has an animated illustration showing how ELISA can be used to detect HIV.

If you want to know more about oen of its discoverers, Eva Engvall, Joe Palca from National Public Radio has written a feature story.