Despite several adverse transfusion effects attributed to leucocytes in platelet transfusions, the main reason to remove leucocytes from transfusions is to reduce HLA alloantibody formation resulting in immunological platelet refractoriness and febrile transfusion reactions due to HLA antibodies in the recipient. When present above a critical level, leucocytes always induce an immune response. This will not always result in HLA antibody formation or the induction of cytotoxic effector cells. Often the result is immune suppression and tolerance towards donor HLA antigens. The immunological effect, either antibody formation or the induction of tolerance, is the result of various distinguished mechanisms. These will be discussed separately, although in transfusion practice these factors are interacting.