The Rh (Rhesus) blood group system (including the Rh factor) is one of thirty-two current human blood group systems. Clinically, it is the most important blood group system after ABO. At present, the Rh blood group system consists of 50 defined blood-group antigens, among which the five antigens D, C, c, E, and e are the most important. The commonly used terms Rh factor, Rh positive and Rh negative refer to the D antigen only. Besides its role in blood transfusion, the Rh blood group system—specifically, the D antigen—is used to determine the risk of hemolytic disease of the newborn (or erythroblastosis fetalis) as prevention is key.
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