Influence of antiserum avidity on inhibition reactions.

The presence in each antiserum of different antibody components, with different binding affinities, quantitatively affects the outcome of an inhibition reaction. A detailed analysis permits the recognition of the following conditions. (a) At high antigen concentrations, the antiserum is approaching saturation; the only effect of adding unlabelled antigen is competition with the labelled antigen for the available antibody sites. (b) At intermediate antigen concentrations, only a fraction of the antibody components, those of higher affinity, are contributing to the binding reaction; in addition to competition, unlabelled antigen produces an increase in the number of reacting antibody sites. The resultant of these two counteracting effects can be predicted according to the slope of a simple linear function. (c) Lastly, at low antigen concentrations, the added antigen does not have any significant inhibitory effect. As a result of this analysis, practical indications are available for selecting antisera for radioimmunoassays on the basis of their avidity parameters, for choosing the most suitable antigen range for each antiserum, and for interpreting the shape of inhibition curves in terms of the avidity and heterogeneity of the antiserum.