Facts About Blood and Blood Types

Blood types are determined by the presence or absence of certain antigens – substances that can trigger an immune response if they are foreign to the body. Since some antigens can trigger a patient's immune system to attack the transfused blood, safe blood transfusions depend on careful blood typing and cross-matching.There are four major blood groups determined by the presence or absence of two antigens – A and B – on the surface of red blood cells. In addition to the A and B antigens, there is a protein called the Rh factor, which can be either present (+) or absent (–), creating the 8 most common blood types (A+, A-, B+, B-, O+, O-, AB+, AB-).

Blood Groups

The giving of blood and its subsequent safe transfusion into a patient is now commonplace. Successful transfusion would not, however, be possible without the realization that the cells and tissues of individuals are distinct and that introduction of blood of one individual into another may cause an adverse reaction with subsequent destruction of the donated cells. Such reactions come about because there are substances — so-called blood group antigens — on the surface of the cells of the blood, especially on the red cells, or erythrocytes, which may interact with antibodies in another person's blood, leading to red cell destruction.There are four main blood groups (types of blood) – A, B, AB and O.

The ABO system

In the ABO system, the surface antigens of the red cells are determined by three genes, A, B, and O.All red cells have on their surface a glycolipid substance called H substance. The A gene and the B gene convert H substance into substance A and B, giving rise to cells of the A and B groups respectively. The O gene has no effect on H substance and thus group O cells have only H substance on their surface. These three genes combine in pairs to give six possible genotypes, AA, AO, BB, BO, AB, and OO. Since A and B are dominant over O, this results in four phenotypes: A, B, AB and O .There are four main blood groups defined by the ABO system: