It has proved possible to develop a novel non-separation fluoroimmunoassay for thyroxine employing antibodies both to thyroxine and to fluorescein incorporated into immune complexes by addition of a common species-specific anti-immunoglobulin G serum. The labelled hapten could bind to antibody of either specificity but not to both at the same time. Binding to anti-fluorescein markedly reduced fluorescence whereas binding to anti-thyroxine did not. Unlabelled thyroxine competed with the labelled hapten only for anti-thyroxine binding sites and, as a result, more of the labelled thyroxine was bound to anti-fluorescein with a decrease in total fluorescence.
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