Alloimmunisation to HLA antigens following transfusion with leucocyte-poor and purified platelet suspensions.

24 previously non-transfused patients were given three transfusions of 200 X 10(8) platelets at 14-day intervals. Group I (12 patients) received leucocyte-poor platelet suspensions with a mean contamination of 15 X 10(6) leucocytes per transfusion. Group II (12 patients) received platelets with less than 5 X 10(6) leucocytes per transfusion. 5 patients in group I and no patients in group II developed lymphocytotoxic antibodies (p = 0.037). Platelets with fewer than 5 X 10(6) leucocytes did not seem to stimulate a response to major histocompatibility antigens, but with a small contaminating dose of leucocytes appeared to be highly immunogenic.