Fluorescence Energy Transfer Immunoassays

Abstract

Homogeneous immunoassays have gained broad acceptance during the past decade and are being widely utilized for both quantitative and qualitative analysis of a wide variety of analytes in the clinical laboratory. Concurrent to the development of enzyme immunoassays, homogeneous fluorescence immunoassays have played an equally important role in the development of a wide variety of immunochemical methods. One of the earliest procedures of histochemical staining using fluorescent labels (Coones, Creech and Jones, 1941) is still a method of choice for immunochemical localization of tissue antigens. After incubation of tissue antigens with fluorescer-labeled primary or secondary antibody, the excess of the labeled reagent is washed away and the resulting stained immune complexes can be visualized under a dark field microscope. This method, although cumbersome as an immunoassay technique, does provide useful qualitative data. Such use of receptor groups in competitive immunoassays was not further exploited until 1959 when the technique of radioimmunoassays was introduced (Berson and Yallow, 1959). The basic technique involves competition of the analyte with a radio-labeled antigen for a limited number of binding sites. After separation of free and bound antigen and measurement of the radioactivity of these fractions, the quantitation can be achieved by comparison with measurements obtained from a set of known standards. This method provided the ability to determine antigens in solutions quantitatively. Although various modifications have been developed during the past years, the basic technique suffers from the short shelf life of the radioisotopes and the complex labor intensive protocols involving separation of the bound and free fractions prior to measurement. More recently, there has been considerable interest in the use of nonisotopic labels for immunoassays. These have included enzymes (Scharpe, Cooneman and Bloome, 1976; Wisdom, 1976; Van Weeman and Schuurs, 1971), metals (Cais, Dari, Eder, et al, 1977), chemiluminescent groups (Simpson, Campbell and Ryall, 1979), and fluorescent groups (Soini and Hemmila, 1979).