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ABO incompatibility is a reaction of the immune system that occurs if two different and not compatible blood types are mixed together.
A, B, and O are the three major blood types. The types are based on small substances (molecules) on the surface of the blood cells. In people who have different blood types, these molecules act as immune system triggers (antigens).
Each person has a combination of two of these surface molecules. Type O lacks any molecule. The different blood types are:
People who have one blood type form proteins (antibodies) that cause their immune system to react against other blood types. Being exposed to another type of blood can cause a reaction. This is important when a patient needs to receive blood (transfusion) or have an organ transplant. The blood types must be matched to avoid an ABO incompatibility reaction.
Because type O lacks any surface molecules, type O blood does not cause an immune response. This is why type O blood cells can be given to patients of any blood type. People with type O blood are called "universal donors." However, people with type O can only receive type O blood.
Since antibodies are in the liquid part of blood (plasma), both blood and plasma transfusions must be matched to avoid an immune reaction.
McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: Saunders; 2006.
Hoffman R, Benz E, Shattil S, Furie B, Cohen H. Hematology: Basic Principles and Practice. 4th ed. Philadelphia, Pa: Churchill Livingstone; 2004.
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