Blood type is determined by the presence or absence of antigens, molecules that can cause an immune response, such as ABO antigens and Rh factor antigens on erythrocytes.
Blood types are determined by the presence or absence of certain antigens. An antigen is a substance that can cause an immune response within the body. Because of this, using blood with non-compatible antigens can trigger a patient to have a negative reaction during a blood transfusion. If a foreign antigen is found by the body, the body produces an antibody, a blood protein made by the immune system in response to a certain antigen. Antibodies attack the foreign protein. Therefore, safe blood transfusions require careful blood typing and blood-type crossmatching to ensure that the body doesn't attack the donor blood.
There are four major blood types determined by the presence or absence of antigens:
Type A has the A antigens in the red blood cells and anti-B antibodies in the plasma.
Type B has the B antigens in the red blood cells and anti-A antibodies in the plasma.
Type AB has both A and B antigens in the red blood cells but has neither anti-A nor anti-B antibodies in the plasma.
Type O has neither A nor B antigens in the red blood cells but both anti-A and anti-B antibodies in the plasma.
In addition to the A and B antigens, there is a protein called the Rh factor. Rh factor is another antigen that can either be present (+) or absent (–) in red blood cells. This results in eight blood types: A+, A−, B+, B−, O+, O−, AB+, AB−. A person whose blood is Rh negative cannot receive blood from an Rh positive donor because the body creates antibodies to attack the Rh positive cells. But a person whose blood is Rh positive can receive blood from an Rh negative donor. Since the O− blood type has no antigens, it can be given to people with all the other blood types; it's the universal donor. Since the AB+ blood type has neither anti-A nor anti-B antibodies, it can be received by people with all blood types, making it the universal receiver.
Blood Types in Humans
Additionally, there are over 600 other known antigens, which create rare blood types. Some of these are unique to specific racial or ethnic groups. For example, people in the Bombay blood group lack both A and B antigens, so they would generally be classified as type O; however, because they also lack the H antigen, which is often present in people with type O blood, if they're given blood from an O donor, they could get sick.