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Immune Mechanisms of Platelet Refractoriness

Multiple platelet transfusions may prove ineffective in approximately 40% of patients treated for bone marrow aplasia. This condition is known as platelet refractoriness and is diagnosed by evaluating the corrected count increment following platelet transfusion. Immune factors still represent an important cause of platelet refractoriness and, among these, HLA alloimmunization plays the most relevant pathogenetic role. Less important are other specific and non-specific antigens, that can be detected on platelet surface. Several assays have been developed to reveal anti-platelet antibodies, which include the lymphocyte cytotoxicity test, the platelet immunofluorescence assay and the mixed passive hemagglutination assay. It is now clear that leukocytes contaminating the platelet concentrates represent the main cause of HLA alloimmunization which can be prevented by leukocyte depletion to less than 5 × 106 cells per unit. Transfusion of HLA-matched platelets in alloimmunized platelets may be quite effective, but it can also be fairly expensive considering the large number of donors to be typed for HLA in order to find HLA compatible platelets. A more practical approach would be to select the platelet concentrates on the basis of the negative crossmatch.