A ligand-binding @A00470@ that uses a specific @A00394@ or @A00384@, capable of binding to the analyte, to identify and quantify substances. The @A00384@ can be linked to a radiosotope (@R05105@, RIA), or to an @E02159@ which catalyses an easily monitored reaction (enzyme-linked immunosorbent @A00470@, ELISA), or to a highly fluorescent compound by which the location of an @A00394@ can be visualized (immunofluorescene).