Eight different blood types are found within the human population: O positive, O negative, A positive, A negative, B positive, B negative, AB positive and AB negative. Blood type is an inherited factor and is determined by the presence of different antigens, a substance that can trigger an immune system response.
The eight blood types are a significant factor in blood transfusions. Because the different antigens present in the donor blood and the recipient can trigger an immune system response, donated blood must be carefully typed and cross-matched before it is transfused into a potential patient in order to prevent the transfusion from being rejected the host's body.
For the purposes of typing blood for transfusions, the ABO Blood Group separates the different blood types by the presence or absence of the antigens A and B on the surface of red blood cells. The four groups, Group A, Group B, Group AB and Group O, all have the A and B antigens present but in different areas of the blood. For instance, Group A has the A antigen present in red blood cells and the B antigen present only in the blood plasma, whereas Group O has neither antigen A or B present in red blood cells but does have them both present in blood plasma.