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
ABO blood group system ABO blood group system The ABO blood group system is used to denote the presence of one, both, or neither of the A and B antigens on erythrocytes. In human blood transfusions it is the most important of the 36 different blood type (or group) classification systems currently recognized. A very rare (in modern medicine) mismatch in this, or any other serotype, can cause a serious, potentially fatal, adverse reaction after a transfusion, or a contra-indicated immune response to an organ transplant. The associated anti-A and anti-B antibodies are usually IgM antibodies, which are produced in the first years of