ABO incompatibility

Definition

ABO incompatibility is a reaction of the immune system that occurs if two different and not compatible blood types are mixed together.

Causes, incidence, and risk factors

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
• 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

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.

Symptoms

The following are symptoms of transfusion reactions:

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

Signs and tests

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

Treatment

Treatment may include:

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