Simple and complex antibody reactions in radioimmunoassay and the prediction of assay characteristics.

Radioimmunoassays are usually developed empirically since there have been few established rules which cover general RIA behaviour. Evidence is presented which enables the delineation of 2 types of RIA with distinct group characteristics. (1) Simple RIA, conforming to the law of mass-action kinetics, is a result of univalent interaction. Examples of simple RIA include hapten assays and those using a monoclonal antibody (McAb). (2) Complex RIA occurs when antigen reacts multivalently with a polyclonal antiserum (PcAs). The formation of multicomponent complexes between a large molecular weight antigen and a PcAs is demonstrated using gel exclusion chromatography. These complexes are resistant to dissociation and are responsible for higher affinity, greater sensitivity and slower equilibration times compared to simple RIA. The assignment of an assay to either the simple or complex RIA group is dependent upon either the molecular size of the antigen or the use of a McAb. The consequent predictability of RIA behaviour enables a more rational approach to optimal assay design than current theory allows. It is advocated that the inability of a McAb to form multicomponent complexes is a major disincentive to their indiscriminate adoption in RIA.