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ABO incompatibility


ABO incompatability describes an immune reaction that occurs in the body if two blood samples of different, incompatible ABO types are mixed together.

Causes, incidence, and risk factors

A, B, and O are the three major blood types. The types are defined by molecules on the surface of the blood cells. In people of different blood types, these molecules act as antigens -- immune-response triggers.

Each person has a combination of two of these surface molecules. Type O refers to a lack of any molecule. So, the resulting types are type A (AA or AO molecules), type B (BB or BO molecules), type AB, or type O.

People of a given type form antibodies against other types. When exposed to another type of blood, a breakdown reaction occurs. This is most relevant when a patient needs a blood transfusion or transplant. The blood types must be matched to avoid a reaction based on ABO incompatibility.

For example, a patient with type A blood will react against type B or type AB blood. Similarly, a patient with type B blood will react against type A or type AB blood, and patients with type O blood will react against type A, type B, or type AB blood.

Because type O signifies a lack of 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, and people with type O blood are called "univeral donors." However, people with type O can only receive type O.

Since antibodies are present in the blood plasma -- the liquid portion of the blood -- plasma transfusions as well as whole blood transfusions must be matched to avoid causing an immune reaction.


The following are symptoms of transfusion reactions:

  • fever
  • blood in urine
  • back pain
  • feeling of "impending doom"
  • jaundice

Signs and tests

  • laboratory testing of patient's blood and transfused blood shows incompatibility
  • complete blood count (CBC) shows damaged red blood cells, may also show mild anemia
  • bilirubin level is elevated


Treatment involves supportive care with intravenous fluids, antihistamines, steroids, and blood pressure support if necessary.

Expectations (prognosis)

This can be a very serious problem which can even result in death. If supportive care is successful, a full recovery is likely.


  • kidney failure
  • low blood pressure requiring intensive support
  • death

Calling your health care provider

Call your health care provider if you have recently had a blood transfusion or transplant and you experience the symptoms listed above.


Careful cross-matching of blood type prior to transfusion or transplant can prevent this problem.

Update Date: 6/1/2003

Marcia S. Brose, M.D., Ph.D., Division of Hematology/Oncology, Hospital of the University of Pennsylvania, Philadelphia, PA. Review provided by VeriMed Healthcare Network.

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Last updated: Tue, 06 Jan 2009 00:20:03 GMT