Reactions of biotinylated antibodies from patients 1 through 4 with immobilized heparin: PF4 complexes previously saturated with unlabeled (“blocking”) antibody from the same patients. Antibody 1 was blocked by itself and (partially) by antibody 4, but not by antibodies 2 and 3. Antibodies 2 and 3 blocked each other, but not antibody 1 or 4. Antibody 4 blocked itself, antibody 3, and (partially) antibodies 1 and 2. Values (ordinate) are expressed as percent of controls in which normal plasma, rather than patient plasma was used for “blocking” and are the average of triplicate determinations ± 1.0 SD.