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
Rh blood group system Rh blood group system The Rh blood group system is one of thirty-five known human blood group systems. It is the second most important blood group system, after the ABO blood group system. The Rh blood group system consists of 49 defined blood group antigens, among which the five antigens D, C, c, E, and e are the most important. There is no d ("little d") antigen. Rh(D) status of an individual is normally described with a "positive" or "negative" suffix after the ABO type. e.g. Someone who is A Positive has the A antigen and the Rh(D) antigen, whereas