Blood platelets contain at least three antigen systems capable of producing a transfusion reaction. The first is the ABO (ABH) system, also found on RBCs; for that reason the AABB recommends that single-donor platelet units be typed for the ABO group before being transfused. Platelets do not contain the Rh antigen; however, since platelet units may be contaminated by RBCs, it is safer to type for D (Rho) antigen in addition to ABO typing. The majority of investigators believe that ABO-Rh typing is useful only to avoid possible sensitization of the recipient to RBCs and that recipient anti-A or anti-B antibody will not destroy ABO-incompatible platelets. There are platelet-specific antigens (PLA1 group, also called Zwa, and several others) that are found in high incidence and thus rarely cause difficulty. However, PLA1 antibodies have been implicated in a syndrome known as posttransfusion purpura (Chapter 8). Finally, there is the class I HLA-A,B,C group. This tissue compatibility system has been incriminated in patients who become refractory to repeated platelet transfusions (i.e., in whom the platelet count fails to rise) and in those who develop a febrile reaction after platelet administration. Interestingly, it has been reported that there is not a good correlation between the number of platelet transfusions and development of HLA antibodies. Persons who are HLA-compatible (usually siblings) as a rule are able to donate satisfactorily. The HLA-A antigens are also present on WBCs. Finally, other types of antiplatelet antibodies may appear, such as those of chronic idiopathic thrombocytopenia. In this situation, transfused platelets are quickly destroyed, making the transfusion of little value unless the patient is actively bleeding (Chapter 10).