Demonstration of 2 distinct antibody populations in serum from patient 6. (Left) An eluate prepared from platelets following incubation with antibody reacted with platelets in the presence of drug. No detectable platelet-reactive antibody remained in the absorbed serum. (Right) The absorbed serum, but not the eluate, reacted with Qn-HSA. The eluted antibody failed to react with platelets in the absence of drug and the reaction of the absorbed serum with Qn-HSA was completely inhibited by soluble quinine (not shown).