<P> The Rh blood group system (including the Rh factor) is one of thirty - five known human blood group systems . It is the second most important blood group system, after the ABO blood group system . 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 D antigen is used to determine the risk of hemolytic disease of the newborn (or erythroblastosis fetalis) for Rh disease management . </P> <P> Rh derives from rhesus and the terms rhesus blood group system, rhesus factor, rhesus positive and rhesus negative are also used . </P> <P> An individual either has, or does not have, the "Rh factor" on the surface of their red blood cells . This term strictly refers only to the most immunogenic D antigen of the Rh blood group system, or the Rh − blood group system . The status is usually indicated by Rh positive (Rh+ does have the D antigen) or Rh negative (Rh − does not have the D antigen) suffix to the ABO blood type . However, other antigens of this blood group system are also clinically relevant . These antigens are listed separately (see below: Rh nomenclature). In contrast to the ABO blood group, immunization against Rh can generally only occur through blood transfusion or placental exposure during pregnancy in women . </P> <P> The rhesus blood type named after the rhesus monkey was first discovered in 1937 by Karl Landsteiner and Alexander S. Wiener . The significance of the discovery was not immediately apparent and was only realized in 1940, after subsequent findings by Philip Levine and Rufus Stetson . This serum that led to the discovery was produced by immunizing rabbits with red blood cells from a rhesus macaque . The antigen that induced this immunization was designated by them as Rh factor to indicate that rhesus blood had been used for the production of the serum . </P>

Where does the rh negative factor come from