<P> The Rh system (Rh meaning Rhesus) is the second most significant blood - group system in human - blood transfusion with currently 50 antigens . The most significant Rh antigen is the D antigen, because it is the most likely to provoke an immune system response of the five main Rh antigens . It is common for D - negative individuals not to have any anti-D IgG or IgM antibodies, because anti-D antibodies are not usually produced by sensitization against environmental substances . However, D - negative individuals can produce IgG anti-D antibodies following a sensitizing event: possibly a fetomaternal transfusion of blood from a fetus in pregnancy or occasionally a blood transfusion with D positive RBCs . Rh disease can develop in these cases . Rh negative blood types are much less common in Asian populations (0.3%) than they are in European populations (15%). The presence or absence of the Rh (D) antigen is signified by the + or − sign, so that, for example, the A − group is ABO type A and does not have the Rh (D) antigen . </P> <P> As with many other genetic traits, the distribution of ABO and Rh blood groups varies significantly between populations . </P> <P> Thirty - three blood - group systems have been identified by the International Society for Blood Transfusion in addition to the common ABO and Rh systems . Thus, in addition to the ABO antigens and Rh antigens, many other antigens are expressed on the RBC surface membrane . For example, an individual can be AB, D positive, and at the same time M and N positive (MNS system), K positive (Kell system), Le or Le negative (Lewis system), and so on, being positive or negative for each blood group system antigen . Many of the blood group systems were named after the patients in whom the corresponding antibodies were initially encountered . </P> <P> Transfusion medicine is a specialized branch of hematology that is concerned with the study of blood groups, along with the work of a blood bank to provide a transfusion service for blood and other blood products . Across the world, blood products must be prescribed by a medical doctor (licensed physician or surgeon) in a similar way as medicines . </P>

Why is type o blood the universal donor