High Quality Content by WIKIPEDIA articles! The Rh (Rhesus) blood group system (including the Rh factor) is one of thirty 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 5 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, the D antigen, in particular, is a relevant cause of the hemolytic disease of the newborn or erythroblastosis fetalis for which prevention is key.