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. 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 fight against type B blood. In the United States, about 40% of the white population, 27% of African Americans, 28% of Asians, and 16% of Native Americans are type A.
• The B antigen, you have type B blood. Your plasma has antibodies that fight against type A blood. In the U.S., about 11% of the white population, 20% of African Americans, 27% of Asians, and 4% of Native Americans are type B.
• Neither the A nor B antigen, you have type O blood. Your plasma has antibodies that fight against both type A and type B blood. In the U.S., about 45% of the white population, 49% of African Americans, 40% of Asians, and 79% of Native Americans are type O.
• Both the A and B antigens, you have type AB blood. Your plasma does not have antibodies against type A or type B blood. In the U.S., about 4% of the white population, 4% of African Americans, 5% of Asians, and less than 1% of Native Americans are type AB.

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.