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Blood Type Test

Blood type tests are done before a person gets a blood transfusion and to check a pregnant woman's blood type. Human blood is typed by certain markers (called antigens) on the surface of red blood cells. Blood type may also be done to see if two people are likely to be blood relatives.

The most important antigens are blood group antigens (ABO) and the Rh antigen, which is either present (positive, +) or absent (negative, -). So the two most common blood type tests are the ABO and Rh tests.

ABO test

The ABO test shows that people have one of four blood types: A, B, AB, or O. If your red blood cells have:

  • The A antigen, you have type A blood. The liquid portion of your blood (plasma) has antibodies that attack type B blood. About 42% of people (42 in 100) in the United States have type A blood, with 6% having A-negative (A-) blood and 36% having A-positive (A+) blood.1
  • The B antigen, you have type B blood. Your plasma has antibodies that attack type A blood. About 10% of people (10 in 100) in the U.S. have type B blood, with 2% having B-negative (B-) blood and 8% having B-positive (B+) blood.1
  • Neither the A nor B antigen, you have type O blood. Your plasma has antibodies that attack both type A and type B blood. About 44% of people (44 in 100) in the U.S. have type O blood, with 7% having O-negative (O-) blood and 37% having O-positive (O+) blood.1
  • Both the A and B antigens, you have type AB blood. Your plasma does not have antibodies against type A or type B blood. About 4% of people (4 in 100) in the U.S. have type AB blood, with 1% having AB-negative (AB-) blood and 3% having AB-positive (AB+) blood.1

Blood received in a transfusion must have the same antigens as yours (compatible blood). If you get a transfusion that has different antigens (incompatible blood), the antibodies in your plasma will destroy the donor blood cells. This is called a transfusion reaction, and it occurs immediately when incompatible blood is transfused. A transfusion reaction can be mild or cause a serious illness and even death.

Type O-negative blood does not have any antigens. It is called the "universal donor" type because it is compatible with any blood type. Type AB-positive blood is called the "universal recipient" type because a person who has it can receive blood of any type. Although "universal donor" and "universal recipient" types may be used to classify blood in an emergency, blood type tests are always done to prevent transfusion reactions.

Minor antigens (other than A, B, and Rh) that occur on red blood cells can sometimes also cause problems and so are also checked for a match before giving a blood transfusion.

WebMD Medical Reference from Healthwise

Last Updated: December 30, 2011
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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