Indirect quenching fluororeceptor assay of anti-AChR antibodies.

A fluororeceptor assay (FRA) has been developed for the determination of antibodies against acetylcholine receptor (AChR), employing an antifluorescein serum and fluorescein-labeled AChR. Antiserum raised against rat muscle AChR in rabbit and the labeled AChR are incubated with antifluorescein serum at room temperature. At high levels of anti-AChR, binding of the labeled AChR prevented subsequent binding of the fluorescein groups by antifluorescein, resulting in little change in the signals of the label. Conversely, at low levels of anti-AChR, the free fraction of the labeled AChR is available to be bound by antifluorescein, which markedly reduced fluorescence intensity of the label. Thus, the fluorescence intensity of the assay mixture directly reflects the amount of anti-AChR antibodies in the serum. It is concluded that the availability of fluorescein-labeled AChR and the antibody directed against it permit measurement of anti-AChR antibodies in human myasthenia. The quality of the assay and its preliminary clinical application have been evaluated.