Isoantibody specificity in post-transfusion purpura.

Antiplatelet antibody in the serum of a fatal case of post-transfusion purpura showed specificity for the Pl A1 platelet isoantigen in that the antibody was adsorbed by all Pl A1 -positive platelets tested but not by Pl A1 -negative platelets. However, some Pl A1 -positive platelets that fixed complement with the reference anti-Pl A1 did not fix complement with this patient’s antibody. When used with these platelets, the patient’s antibody competitively blocked the complement-fixing activity of reference anti-Pl A1 . There was no evidence for lack of antigenic sites on the noncomplement-fixing platelets or for antibodies with more than one specificity in the patient’s serum. These studies, therefore, indicated that the complement-fixing activity of different examples of anti-Pl A1 is variable, and that this variation is probably due to distribution of Pl A1 antigenic sites on cell surfaces rather than to differences in antigenic structure that affect antibody affinity.