Even type O blood contains proteins that may cause a transfusion reaction.
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All blood might look the same when it's pouring from a cut. But blood cells contain a number of different genetically determined proteins, or antigens. These antigens can trigger reactions during a blood transfusion. Therefore, blood cell antigens are the primary determinants of what blood type can be safely given to another person for a transfusion. Only O-negative blood is considered compatible with all other blood types, and even this is not always true.
ABO Blood Type
Your blood is one of four types -- A, B, AB or O. This is called the ABO blood typing system. Blood typing is based on the presence or absence of blood cell antigens A and B. If you have type A blood, you have A antigens. If you have type B blood, you have B antigens. If you have both A and B antigens, you have type AB blood. If your blood type is O, you have neither antigen A nor B.
Your ABO blood type also determines what antibodies you have in your blood against other blood types. These antibodies are special proteins that react with blood antigens A or B. If you have type A blood, your blood also contains antibodies to the B antigen. If you have type B blood, you have antibodies to type A blood. If you have type AB blood, you do not have any A or B antibodies. But if you have type O blood, you have antibodies against both the A and B antigens. ABO antibodies are critical in determining what type of blood you can receive for a transfusion. For example, if you have type A blood and receive type B blood, your B antibodies will attack the B antigens in the transfused blood, causing a severe transfusion reaction. Because type O blood does not contain A or B antigens, people with any of the four major blood types can receive it.
The Rh Factor
The ABO system is not the only important factor in blood typing. The Rh factor -- another type of blood cell antigen -- also plays a part in whether your body will accept blood without a reaction. If you have the Rh factor, you are Rh-positive. If you do not have the Rh factor, you are Rh-negative. Approximately 85 percent of the population is Rh-positive, according to "Brunner and Suddarth's Textbook of Medical-surgical Nursing." Although your body does not naturally make antibodies to the Rh factor if you are Rh-negative, exposure to Rh-positive blood will trigger production of these antibodies. The presence of these Rh antibodies could create problems with a future pregnancy or transfusion. Therefore, only O-negative blood can be used as a universal donor. O-positive blood can still be given to people with any of the major ABO types, as long as they are also Rh-positive.
In most cases, O-negative blood can be safely given to anyone because it does not contain any of the major antigens that can trigger a severe immune response, known as a transfusion reaction. In a transfusion reaction, antibodies in the recipient's bloodstream attack antigens on the donor's red blood cells. This destroys the red blood cells and causes a severe immune response. Symptoms include anxiety, fever, chills, low back pain, pain at the infusion site and difficulty breathing. Shock may develop if the transfusion is not stopped immediately. Transfusion reactions are generally more severe with ABO incompatibilities than with Rh incompatibility.