A technique that uses a fluorochrome to indicate the occurrence of a specific antigen-antibody reaction. The fluorochrome labels either an antigen or an antibody. The labeled reactant is then used to detect the presence of the unlabeled reactant, for example, a component of a section of body tissue seen under a microscope. The use of a labeled reactant (such as an antibody which both detects and indicates the antigen) to reveal the presence of an unlabeled one is termed direct immunofluorescence. The use of a labeled indicator antibody, which reacts with an unlabeled detector antibody that has previously reacted with an antigen, is termed indirect immunofluorescence. Both techniques can be used qualitatively to indicate the presence of a specific reactant in a preparation. They can also be used semiquantitatively, for example, to determine the highest dilution (lowest concentration) of a tested detector reactant that can detect a given substance. Substitution of a light meter for the human eye permits a quantitative measurement in immunofluorometry.