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A antigen is a protein which is present on the surface of red blood cells. This antigen is part of the ABO blood group system, which is the most important blood group system taken into account when someone receives a blood transfusion. The ABO blood group system was discovered in 1900 by Austrian scientist Karl Landsteiner, who later received a Nobel Prize for his work.
Landsteiner’s discovery was the existence of three blood types, classified on the basis of the presence of antigens he described as A, B, and O. In 1902, another research group discovered the existence of a fourth type, called AB, with red blood cells which express both A and B antigens. A third group determined in 1910 that ABO blood type was an inheritable trait.
A person who with red blood cells which express A antigen on their surfaces is said to have type A blood. Someone who has B antigen has type B blood, while someone with both A and B antigen has type AB blood. People with type O blood express neither A antigen nor B antigen.
Within the A antigen there are around twenty different subtypes. Most of these subtypes are extremely rare. More than 99% of people have either A1 or A2; the A1 subtype is the most common, and is present in around 80% of people. The A1 and A2 subtypes are so similar that they do not need to be distinguished for the purposes of transfusion. Some of the other A subtypes are different enough to complications when blood is being typed, but these subtypes are so rare that such issues arise very infrequently.
For the most part, ABO antigen differences are important only in situations where an individual requires a blood transfusion. Before someone receives a blood transfusion, the medical staff giving the transfusion must know the blood type of the recipient. This is because the immune system generates antibodies to ABO antigens that are not expressed by red blood cells.
In the case of someone with type A blood, for example, that person’s red blood cells express A antigen. The immune system of that person generates antibodies to B antigen early in life, usually in the first year or two. The end result is that if someone with type A blood receives a blood transfusion from someone with type B blood, antibodies from the recipient’s immune system will destroy the transfused red blood cells.
Therefore, a transfusion recipient with type A blood must receive a transfusion of either type A or type O blood. This is because these are the only blood types that will not provoke an immune response. Similarly, type A blood can only be donated to someone with type A or AB blood, due to the presence of antidotes to the A type antigen in people with type B or type O blood.