Alloimmunization against the MHC antigens after platelet transfusions is due to contaminating leukocytes in the platelet suspension.

Repeated platelet transfusion to thrombocytopenic patients frequently induce anti HLA antibodies, which are responsible for transfusion refractoriness. As the transfused platelet suspensions usually contain 15-30% of the leukocytes originally present in the blood, it is not known whether these antibodies are raised by the platelets or by the contaminating leukocytes in the platelet suspensions. In the mouse, pure platelet suspensions are not able to induce a primary antibody response, as measured by the NIH test and the indirect immunofluorescence test on platelets and leukocytes, despite repeated injections. However, when the platelet suspensions are contaminated with leukocytes (10(3) or more/injection) an antibody response is induced. This response is higher than the response indiced by an equal amount of leukocytes alone. As in man the use of leukocyte poor platelets postpones the development of refractoriness to random platelets it is concluded that transfusions with leukocyte free platelets will probably prevent immunization against the HLA antigens.