Enzyme-linked immunosorbent assay (ELISA) is a highly sensitive technique for detecting and measuring antigens or antibodies in a solution.
In this assay, the solution is run over a surface to which immobilized antibodies specific to the substance in the solution have been attached. If the substance of interest is present, it will bind to the antibody layer, and its presence is verified and visualized with an application of antibodies that have been tagged in some way, usually with a radioactive or fluorescent marker.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.