Packed red blood cells do not have an antibody, since the greatest risk of Rh incompatible blood is to current or future pregnancies. For RBCs, type O negative blood is considered a "universal donor" as recipients with types A, B, or AB can almost always receive O negative blood safely. Type AB positive is considered a "universal recipient" because they can receive the other ABO/Rh types safely. These are not truly universal, as other red cell antigens can further complicate transfusions. There are many other human blood group systems and most of them are only rarely associated with transfusion problems. A screening test is
Blood doping cells into surrounding tissue during each passage through the cardiovascular system. ITPP has been a subject of anti-doping research in both humans and racehorses. Blood transfusions can be traditionally classified as autologous, where the blood donor and transfusion recipient are the same, or as allogeneic/homologous, where the blood is transfused into someone other than the donor. Blood transfusion begins by the withdrawal of 1 to 4 units of blood (1 unit = 450 ml of blood) several weeks before competition. The blood is centrifuged, the plasma components are immediately reinfused, and the corpuscular elements, principally red blood cells (RBCs), are