<P> The Rh blood group system consists of nearly around 50 different antigens, but the one of the greatest clinical interest is the "D" antigen, though it has other names and is commonly just called "negative" or "positive ." Unlike the ABO antigens, a recipient will not usually react to the first incompatible transfusion because the adaptive immune system does not immediately recognize it . After an incompatible transfusion the recipient may develop an antibody to the antigen and will react to any further incompatible transfusions . This antibody is important because it is the most frequent cause of hemolytic disease of the newborn . Incompatible red blood cells are sometimes given to recipients who will never become pregnant, such as males or postmenopausal women, as long as they do not have an antibody, since the greatest risk of Rh incompatible blood is to current or future pregnancies . </P> <P> 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 . </P> <P> There are many other human blood group systems and most of them are only rarely associated with transfusion problems . A screening test is used to identify if the recipient has any antibodies to any of these other blood group systems . If the screening test is positive, a complex set of tests must follow to identify which antibody the recipient has by process of elimination . Finding suitable blood for transfusion when a recipient has multiple antibodies or antibodies to extremely common antigens can be very difficult and time - consuming . </P> <P> Because this testing can take time, doctors will sometimes order a unit of blood transfused before it can be completed if the recipient is in critical condition . Typically two to four units of O negative blood are used in these situations, since they are unlikely to cause a reaction . A potentially fatal reaction is possible if the recipient has pre-existing antibodies, and uncrossmatched blood is only used in dire circumstances . Since O negative blood is not common, other blood types may be used if the situation is desperate . </P>

Packed red blood cells with adenine saline added