Published on July 17, 2010.
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 are called universal donors, because if their blood has none of the antigens, it can be combined with any blood type without the risk of an immune response. On the other hand, people with Type AB are universal receivers, because their immune system does not respond to any of the antigens.
Since blood type is a generic trait, a parent with Type A can have children with Type A or Type B, but not AB or O. Research indicates that blood type may be connected to susceptibility or vulnerability to certain diseases. According to this research, people with Type B or O have a slightly lower risk of certain cancers, while those with A or B antigens, or both, have a lower risk of cholera or plague. It is possible that the antigens developed as part of the body ‘s immune response to diseases.