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Most adverse blood transfusion (BT) events are immune-mediated and in the majority of severe reactions antibodies can be identified as causal factors. Alloimmunization not only causes symptomatic reactions, transfused cells can also be (silently) destroyed. Immunization by BT can contribute to hemolytic disease of the newborn as well as to allograft rejection after transplantation. Reversely, pregnancy and transplantation may evoke immunity hampering transfusion therapy. Besides causing mortality and morbidity, alloimmunization has a huge economic impact. Transfusion reactions prolong hospital stay, require diagnostic tests and complex donor selection procedures and create the need for typed donor registries. In the 1970s, Opeltz and colleagues described that pre-transplantation BT impaired rejection of renal transplants. Leukocytes were essential for this immunosuppressive BT effect that raised concern about negative effects on cancer growth and resistance against infections. Studies on the mechanism were however preliminary abandoned when calcineurin inhibitors for prevention of graft rejection became available and since all blood products underwent leukoreduction in most countries as precautionary measure against transmission of variant Creutzfeldt-Jacob disease. Whether current leukoreduced BT are immunosuppressive and for which patients or circumstances this may contribute to worse outcome, is unknown. The last decades of the previous century, leukoreduction of cellular blood products for leukemia patients significantly reduced the incidence of immunological platelet transfusion refractoriness. The first decade of this century the avoidance of plasma- and platelet-products from females, that may contain donor-derived leukocyte antibodies, decreased transfusion related acute lung injury (TRALI) by more than 30%. These were major achievements. Challenge for the near future is to further reduce alloimmunization in particular against red blood cells (RBC) as a cause of severe hemolytic transfusion reactions and problems to find compatible donors. This can be achieved by extended matching. Inventory limitations prevent to match all BT for clinically relevant RBC antigens. A (European-wide) registration of clinical and genetic risk factors associated with alloimmunization could support effective use of matched blood products.Le texte complet de cet article est disponible en PDF.