Transfusion of erythrocytes is the most common form of cellular therapy. It rapidly restores the oxygen supply to the tissues in patients suffering from sudden blood loss, reduced erythrocyte production following for instance cytotoxic cancer treatment, or in patients suffering from chronic anemia due to congenital diseases such as sickle cell disease and thalassemia. Matching transfusions for ABO and Rh-D blood group antigens is in general sufficient for successful transfusion. However, over 300 additional blood group antigens are known, against which we do not normally generate antibodies, and that are not matched for in general transfusion practice.