ELISA

ELISA (enzyme-linked immunosorbent assay), invented by Engvall and Perlman in 1971, measures antigens or antibodies in a sample. An enzyme is attached to an antibody. When a substrate is added, the enzyme turns it into a coloured product, and an ELISA reader measures the colour. The more colour, the more of the target. Comparing with a standard curve (made with known amounts) gives the exact concentration, not just yes or no.

There are four main types:

• Direct ELISA: the antigen is coated on the well and detected by one enzyme-labelled antibody. Quick, but more likely to give non-specific binding.
• Indirect ELISA: antigen on the well; an unlabelled primary antibody binds it; then an enzyme-labelled secondary antibody binds the primary. More sensitive, because several labelled antibodies can bind each primary one, amplifying the signal.
• Sandwich ELISA: a capture antibody coated on the well holds the antigen; a second, enzyme-labelled antibody binds another part of it. The antigen is "sandwiched" between two antibodies. Very specific, and the antigen need not be purified first.
• Competitive ELISA: antigen in the sample and antigen on the well compete for a limited amount of antibody. The more antigen in the sample, the less antibody is left to bind the well, so more antigen gives less colour.

Uses: detecting antigens or antibodies; measuring antibodies in blood when testing for viruses; finding food allergens; and tracking disease outbreaks such as HIV, bird flu and cholera.