Use of antibodies against the label in non-separation non-isotopic immunoassay: 'indirect quenching' fluoroimmunoassay of proteins.

Antibodies against the label are introduced as a potentially useful reagent in nonisotopic immunoassay. They may permit end point determination without the need for a separation step, provided (i) that steric hindrance selectively prevents their binding to the antibody-bound fraction of the labelled antigen in an immunoassay mixture, and (ii) that their binding to the label in the free fraction results in a change in its signal. This 'indirect' approach was investigated in systems employing the fluorescein label and antibodies to fluorescein which quenched the fluorescence of free labelled antigen. 'Indirect quenching' fluoroimmunoassays for human serum albumin, human immunoglobulin G and human placental lactogen were demonstrated. These assays for proteins may be contrasted with conventional non-separation techniques, which are usually best suited to the determination of haptens.