The discrepancy between the inhibitory activity induced on passive hemagglutination by the free and conjugated hapten. The significance of the phenomenon for the explanation of the drug allergy.

The capacity of the free and conjugated haptens to inhibit passive hemagglutination has been comparatively studied in five immunological systems, using hapten specific antisera prepared in rabbits and conjugates of the respective haptens with rabbit serum proteins. The results demonstrate that the affinity of the free hapten for the specific antibodies is 100 to 10,000 times lower than the affinity of the conjugated hapten. This finding constitutes an explanation for the occurrence of the allergic drug reactions under the circumstances where the concentrations of the free hapten are much higher than the concentrations of the hapten bound in in vivo formed conjugates. Using the passive hemagglutination inhibition test, the affinity of the various forms of a given hapten for the specific antibodies can be comparatively evaluated.