ABO incompatibility

Mixing two blood samples of different ABO types, resulting in an immune reaction against the foreign blood cells.

Causes, incidence, and risk factors

A, B and O are the three major blood types. These types are defined by molecules on the surface of the blood cells which act as antigens, or inducers of immune responses in people of different blood types. Each person has two types of these molecules, in any combination. Since type O refers to a lack of the antigenic molecule, 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 blood type form antibodies against other blood types, causing a hemolytic (blood breakdown) reaction. 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 AB blood. Similarly, a patient with type B blood will react against type A or AB blood, and patients with type O blood will react against type A, type B or type AB blood. Type O blood does not cause an immune response, and therefore type O cells (universal donor) can be given to patients of any blood type. Since antibodies are present in the blood plasma, or liquid portion of the blood, plasma transfusions as well as whole blood transfusions must be matched to avoid inducing this reaction.

Signs and tests

  • cross match in laboratory of patient's blood and transfused blood shows incompatibility
  • CBC
  • shows damaged and hemolyzed red blood cells, may also show spherocytosis and a mild
  • anemia
  • bilirubin
  • level is elevated
  • Treatment

    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.

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