There are four different blood types: Type A, Type B, Type AB (both antigens) and Type O (neither antigen). If blood containing one of the antigens is transfused into a person whose cells don ‘t have that antigen, the immune system treats it as an invading infection and tries to destroy the invaders. Because of those specific antigens, if two wrong blood types get mixed together they form dangerous clumps. Later on, after the ABO system classification developed, a third antigen was discovered. Because it was first observed in the blood of rhesus monkeys, it was called Rh. According to this third type, red cells with the antigen are Rh positive, and those without it, Rh negative. Combined with the ABO system, these classifications give eight distinct blood types. People with Type O ar ...Read More