The Nature of the Antigen-Antibody Complexes which Initiate Anaphylactic Reactions

Abstract

Summary A divalent benzylpenicilloyl (BPO) hapten and several homologous multivalent BPO haptens of widely differing molecular sizes were compared quantitatively with regard to their abilities to evoke PCA reactions in guinea pigs passively sensitized with threshold, optimal and excess amounts of rabbit anti-BPO antibodies. These haptens were compared also with regard to their abilities to specifically precipitate rabbit anti-BPO sera, and to evoke passive Arthus reactions. It was found that equimolar concentrations (and not equal weight concentrations) of the divalent and of the multivalent haptens were equally effective in evoking PCA reactions throughout the dose-response curve. In contrast, equal weight concentrations of the multivalent haptens (and not equimolar concentrations) were found to be equally effective in precipitating anti-BPO sera, and in evoking passive direct Arthus reactions in guinea pigs sensitized with rabbit anti-BPO sera. The divalent hapten was virtually ineffective in precipitating anti-BPO antibodies present in these sera, and failed to evoke passive direct Arthus reactions. These results are interpreted as demonstrating, at least in this system, that, whereas the precipitin reaction and the passive direct Arthus reaction depend on the formation of large cross-linked complexes, the PCA reaction in guinea pigs is initiated by the simple bridging of two (or a small integer (n)) firmly fixed anaphylactic antibody molecules by multivalent haptens to form comparatively simple immune complexes of the composition H1Abn.