ABO incompatibility is an immune system reaction that occurs when blood from two different and incompatible blood types are mixed together.

See also: Transfusion reaction - hemolytic

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 (one A and one B molecule)
• 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
• A patient with type O blood will react against type A, type B, or type AB blood
• A patient with type AB blood will NOT react against type A, type B, or type AB blood

Because type O does not have any surface molecules, type O blood does not cause an immune response based on ABO incompatibility. 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 ABO incompatible transfusion reactions:

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

• Bilirubin level is hight
• Complete blood count (CBC) shows damaged to red blood cells or anemia
• The patient's and donor's blood are not compatible

Treatment may include:

• Drugs used to treat allergic reactions (antihistamines)
• Drugs used to treat swelling and allergies (steroids)
• Fluids given through a vein (intravenously)
• Medicines to raise blood pressure if it drops too low

ABO incompatibility can be a very serious problem that 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 symptoms of ABO incompatibility.

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

Goodnough L. Transfusion medicine. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 183.

Wu YY, Mantha S, Snyder EL. Transfusion reactions. In: Hoffman R, Benz E Jr, Shattil S, et al, eds. Hematology: Basic Principles and Practice. 5th ed. Philadelphia, Pa: Churchill Livingstone; 2008:chap 153.