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Why do people have different types of blood?

Wendy Sisson, clinical instructor of nursing, Eleanor Mann School of Nursing in the College of Education and Health professions replies:

Your blood type is determined before birth when you inherit DNA from both parents. The inherited genetic information determines which antigens are present on the surface of your red blood cells. An antigen is a protein that is capable of instigating the formation of antibodies. There are four blood types: A, B, AB and O. Tests to determine blood type detect which antigens are present in blood. Red blood cells of the A type have A antigen on them. B red blood cells have B antigen. If both antigens are detected, the blood type is AB. If no antigens are detected, the blood is type O.

Blood groups are also classified by the Rh factor. The Rh factor is another antigen that is present in some people’s blood. Blood can either have Rh antigen — Rh positive — or have no Rh antigen — Rh negative.

It is important to know your blood type and Rh status if you must receive a transfusion or if you are pregnant. If you are given a transfusion of blood that contains antigens not already present in your blood, your body will form antibodies to the foreign antigen. The antibodies cause blood to clot and can destroy red blood cells. If an Rh negative mother carries an Rh positive baby and the blood of mother and baby mix, the antibodies formed by the mother can destroy the baby’s red blood cells. Rh negative pregnant women are given a medication to prevent this reaction.

The rarest blood type is AB-. Type O- blood is the “universal donor” because transfusions of this type of blood can be given to people with any blood type. Type AB+ is the “universal recipient” because people with this type can receive transfusions of any other type of blood.

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