Binding of antibody from patient JH to normal platelets at various concentrations of histamine, ranitidine, and three other H₂R antagonists. Ratio of the fluorescent signal obtained in the presence of drug to that obtained in its absence is shown on the ordinate. Of the compounds tested other than ranitidine, only nizatidine promoted antibody binding. Values shown are the averages of duplicate determinations, which did not vary from one another by more than 5%.