A system of describing the oligosaccharide antigens found on the surface of human blood cells. According to the type of antigen present, a person may be assigned a blood type of A, B, AB or O. A second type of antigen, the Rh factor, renders a "positive" or "negative" blood type. The ABO blood group system is important because it determines who can donate blood to or accept blood from whom. Type A or AB blood will cause an immune reaction in people with type B blood, and type B and AB blood will cause a reaction in people with type A blood. Conversely, type O blood has no A or B antigens, so people with type O blood are "universal donors." And since AB blood already produces both antigens, people who are type AB can accept any of the other blood types without suffering an immune reaction. The ABO system is also important because it can be used in paternity suits to rule out whether a man is the father of a certain child or not.