The alloimmunized patient: effective transfusion support and newer experimental approaches.

Refractoriness occurs in a proportion of patients who receive multiple platelet transfusions. Alloantibodies, in particular those directed against the class I human leucocyte antigens (HLA) present on the platelet surface, are frequently associated with accelerated platelet destruction and transfusion failure. Compatible platelets for transfusion may be found by selecting donations from HLA-typed individuals, crossmatching patients' sera against donor lymphocytes and platelets or both. Maintaining a large panel of HLA-typed apheresis donors is expensive and some mismatching is inevitable due to the polymorphic nature of the HLA system. Cross-matching may improve the outcome when HLA-matched donations are transfused and may also be a cost-effective strategy for the selection of compatible untyped platelets. Administration of intravenous immunoglobulin, plasma exchange and ex vivo removal of platelet HLA have been employed successfully in a few refractory alloimmunized patients but do not yet have an established role. Experimental studies in animal models suggest that it may be possible to induce tolerance to HLA and prevent alloimmunization.