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 factor is not identical to the rhesus monkey factor; but by then, "Rhesus Group" and like terms were already in widespread, worldwide use. Thus, notwithstanding it is a misnomer, the term survives (e.g. "rhesus blood group system", and the obsolete terms "rhesus factor", " rhesus positive", and "rhesus negative" - all three which actually refer specifically and "only" to the Rh D factor; and, thus, are misleading when unmodified). Contemporary practice is to use "Rh" as a term of art instead of "Rhesus"; viz. "Rh Group," "Rh factors," "Rh D," etc. The first rhesus blood type was discovered in 1937