The Rh blood group system is a human blood group system. It contains proteins on the surface of red blood cells. After the ABO blood group system, it is the most likely to be involved in transfusion reactions. The Rh blood group system consisted of 49 defined blood group antigens in 2005. As of 2023, there are over 50 antigens among which the five antigens D, C, c, E, and e are the most important. There is no d antigen. Rh status of an individual is normally described with a positive or negative suffix after the ABO type antigen, whereas someone who is A− has the A antigen but lacks the Rh antigen). The terms Rh factor, Rh positive, and Rh negative refer to the Rh antigen only. Antibodies to Rh antigens can be involved in hemolytic transfusion reactions and antibodies to the Rh and Rh antigens confer significant risk of hemolytic disease of the newborn.