A blood transfusion is the transfer of blood or a blood component from one person (a donor) to another (a recipient).

In the United States, about 29 million blood transfusions are given every year. Transfusions are given to increase the blood's ability to carry oxygen, restore the amount of blood in the body (blood volume), and correct clotting problems. People who have been injured, people undergoing surgery, and people receiving treatment for cancers (such as leukemia) or other diseases (such as the blood diseases sickle cell anemia and thalassemia) are typical recipients.

The Food and Drug Administration (FDA) strictly regulates the collection, storage, and transportation of blood and its components. These regulations were developed to protect both the donor and the recipient. Additional standards are upheld by many state and local health authorities, as well as by organizations such as the American Red Cross and the American Association of Blood Banks. Because of these regulations, giving and receiving blood is very safe. However, transfusions still pose risks for the recipient, such as allergic reactions, fever and chills, excess blood volume, and bacterial and viral infections. Even though the chance of contracting AIDS, hepatitis, or other infections from transfusions is remote, doctors are well aware of these risks and order transfusions only when there is no alternative. Before ordering a transfusion (except in an emergency), doctors explain the risks of transfusion to people and ask them to sign a document affirming that they understand the risks and giving their consent for transfusion (called informed consent).

Blood typing: People have different blood types. Blood type is determined by whether certain proteins (Rh factor and blood group antigens A and B) are present on the surface of red blood cells. These proteins are considered antigens because they can trigger an immune response.

The four main blood types are A, B, AB, and O

• A: Antigen A (but not B) is present.
• B: Antigen B (but not A) is present.
• AB: Antigens A and B are present.
• O: Neither antigen A nor B is present.

Also, blood may be Rh-positive (Rh factor is present on the surface of the red blood cells) or Rh-negative (Rh factor is absent).

Some blood types are far more common than others. In the United States, the most common blood types are O-positive and A-positive, followed by B-positive, O-negative, A-negative, AB-positive, B-negative, and AB-negative.

Normally, if people lack an A and/or a B antigen, they have naturally occuring antibodies against the antigen or antigens that they lack. For example, people with blood type A have naturally occuring anti-B antibody, and people with blood type O (who lack both A and B antigens) have naturally occuring 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 occuring antibodies against these antigens. Such antibodies develop only if people are exposed to these antigens by transfusion.

Compatible Blood Types 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 (see Blood Transfusion: Destruction of red blood cells. ). 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.

Last full review/revision October 2012 by Ravindra Sarode, MD