blood disorders
Overview of Blood Transfusion
Blood typing
People have different blood types. Blood type is determined by whether certain antigens (complex sugar or protein molecules that can trigger an immune response) are present on the surface of red blood cells. Blood cell antigens include blood group antigens A and B and Rh factor. Thefour main blood typesare A, B, AB, and O (distribution in general population) A: Antigen A (but not B) is present. (40%) B: Antigen B (but not A) is present. (10%) AB: Antigens A and B are present. (5%) O: Neither antigen A nor B is present. (45%) Also, blood may be Rh-positive (Rh factor is present on the surface of the red blood cells, 85% of people) or Rh-negative (Rh factor is absent, 15% of people). Normally, if people lack an A and/or a B antigen, they have naturally occurring antibodies against the antigen or antigens that they lack. For example, people with blood type A have naturally occurring anti-B antibody, and people with blood type O (who lack both A and B antigens) have naturally occurring anti-A and anti-B antibodies. In addition to A and B antigens, there are several other blood group antigens also present on red blood cells. However, people do not have naturally occurring antibodies against these antigens. Such antibodies develop only if people are exposed to these antigens by transfusion. Some blood types are far more common than others. In the United States, the most common blood types in whites are O-positive (37%) and A-positive (33%), followed by B-positive (9%), O-negative (8%), A-negative (7%), AB-positive (3%), B-negative (2%), and AB-negative (1%). A blood transfusion is safest when the blood type of the transfused blood matches the recipient's blood type and Rh status (in other words, the blood types are compatible). Therefore, before a transfusion, blood banks do a test called a type and cross-match on the donor's and the recipient's blood. This test minimizes the chance of a dangerous or possibly fatal reaction. In addition, the recipient's blood is checked for certain antibodies to red blood cells. Such antibodies can cause a reaction to transfused blood. However, in an emergency, anyone can receive type O red blood cells. Thus, people with type O blood are known as universal donors. People with type AB blood can receive red blood cells from a donor of any blood type and are known as universal recipients. Recipients whose blood is Rh-negative must receive blood from Rh-negative donors (except in life-threatening emergencies), but recipients whose blood is Rh-positive may receive Rh-positive or Rh-negative blood. 