Low avidity as a cause of prozone phenomena.

The incidence of prozones was studied, under various conditions, in a model indirect haemagglutination system and in a radioimmunoassay--both assays depending on the use of anti-immunoglobulin (anti-Ig). It was found that prozones were correlated with the presence of low avidity antibodies, and that these could compete with high avidity antibodies for the limited amount of anti-Ig available. It is proposed that the relatively rapid dissociation of low avidity antibodies allows them to form immune aggregates with the anti-Ig. With increasing size these aggregates would become more susceptible to being washed off. In this way low avidity antibodies could occupy the anti-Ig, and yet be relatively ineffective, either for haemagglutination or for the binding of radioactively labelled (or fluorescein-labelled) anti-Ig to the antigen.