Any of various techniques that measure the amount of a particular substance by virtue of its binding antigenically to a specific antibody. In solid-phase immunoassay the specific antibody is attached to a solid supporting medium, such as a PVC sheet. The sample is added and any test antigens will bind to the antibody. A second antibody, specific for a different site on the antigen, is added. This carries a radioactive label (in radioimmunoassay) or fluorescent label (in fluoroimmunoassay), enabling its concentration, and thus that of the test antigen, to be determined by comparison with known standards. Variations on this technique include ELISA and western blotting. In counting immunoassay, polystyrene beads are coated with an excess of the specific antibody. Beads that bind antigen clump together, whereas unbound beads do not and are counted using a cell counter. The proportion of unbound beads is inversely proportional to the concentration of antigen in the sample. The principle of immunoassay has also been employed with certain types of microarray.