Blood type represent contributions from both parents. A total of 36 human blood group systems and 346 antigens are now recognized by the International Society of Blood Transfusion (ISBT). The two most important ones are ABO and the Rh blood group systems; they determine someone's blood type (A, B, AB and O, with +, − or null denoting RhD status) for suitability in blood transfusion. A complete blood type would describe a full set of 30 substances on the surface of red blood cells, and an individual's blood type is one of many possible combinations of blood-group antigens. Across the 36 blood
ABO blood group system it is more likely that the force driving evolution of allele diversity is simply negative frequency-dependent selection; cells with rare variants of membrane antigens are more easily distinguished by the immune system from pathogens carrying antigens from other hosts. Thus, individuals possessing rare types are better equipped to detect pathogens. The high within-population diversity observed in human populations would, then, be a consequence of natural selection on individuals. HIV can be neutralized in "in vitro" experiments using antibodies against blood group antigens specifically expressed on the HIV-producing cell lines. The carbohydrate molecules on the surfaces of red blood cells have