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 type A (AA or AO molecules), type B (BB or BO molecules), type AB, or type O.

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.

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

The following are symptoms of transfusion reactions:

• Fever
• Blood in urine
• Back pain
• Feeling of "impending doom"
• Yellow skin (jaundice)

• Lab testing of patient's and donor's blood shows that they are not compatible
• Complete blood count (CBC) shows damaged red blood cells, may also show mild anemia
• Bilirubin level is high

Treatment may include fluids given through a vein (intravenous), drugs used to treat allergic reactions (antihistamines), drugs used to treat swelling and allergies (steroids), and blood pressure medicines.

This can be a very serious problem which can even result in death. With the right treatment, a full recovery is likely.

• Kidney failure
• Low blood pressure needing intensive care
• Death

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

Careful testing of donor and patient blood types before transfusion or transplant can prevent this problem.

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.