The major human blood type system which describes the oligosaccharide glycoprotein 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. They are termed universal recipients.
To put it all another way a person’s blood group is an inherited feature on the surface of the red blood cells. A series of related blood types constitute a blood group system such as the Rh or the ABO system. In the US, the most common type is O+ which is present in 37.4% of the population. The frequencies in descending order are O+ (37.4%), A+ (35.7%), B+ (8.5%), O- (6.6%), A- (6.3%), AB+ (3.4%), B- (1.5%) and AB- (0.6%). A person can be A, B, AB, or O. If a person has two A genes, their red blood cells are type A. If a person has two B genes, their red cells are type B. If the person has one A and one B gene, their red cells are type AB. If the person has neither the A nor B gene, they are type O. The situation with antibodies in blood plasma is just the opposite of the red cell antigen types. Someone with type A red cells has anti-B antibodies (directed against type B red cells) in their blood. Someone with type B red cells has anti-A antibodies in plasma. Someone who is type O has both anti-A and anti-B antibodies in plasma. And someone who is type AB has neither anti-A nor anti-B antibodies in plasma.