Why Do We Have Different Blood Types?

The ABO blood group system, discovered by Karl Landsteiner in 1901 (earning him the 1930 Nobel Prize), is determined by glycoprotein antigens on red blood cell surfaces. The A and B genes code for enzymes that attach different sugars to a common precursor molecule (H antigen). Type O has no additional sugars (just the H antigen). Your immune system produces antibodies against whichever antigens you lack: type A blood has anti-B antibodies, type B has anti-A, type O has both, and type AB has neither. The evolutionary persistence of multiple blood types suggests they confer selective advantages against different pathogens. Type O appears to offer some resistance to severe malaria (Plasmodium falciparum), which may explain its high frequency in tropical Africa. Type A may increase susceptibility to smallpox, while type B may reduce susceptibility to plague. The Rh factor (positive/negative) involves the D antigen on another protein and is important in pregnancy (Rh incompatibility).