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Blood Typing
A bag of type A positive.

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Blood transfusions were not possible until Karl Landsteiner first identified the major human blood groups -- namely O, A, B, and AB -- in a series of experiments in 1901 that earned him the Nobel Prize. (At the time, Landsteiner identified only groups A, B, and O; further analysis, two years later, revealed group AB.) Group AB recipients can receive blood from all groups.

The ABO blood groups are defined by specific inherited molecules, or antigens, that are present on the surface of red blood cells. Thus, one inherits either A or B antigens (group A or B), both A and B antigens (group AB), or neither antigen (group O). Conversely, a person develops a natural immunity, or antibody, in their plasma to the ABO antigens that are absent on their own red cells. Thus, a group A person has anti-B antibodies, and group O person has both anti-A and anti-B antibodies.

If group A red cells are mistakenly transfused to a group O recipient, for example, the anti-A antibody in the recipient's plasma destroys the transfused group A cells and a serious transfusion reaction occurs. Because group O has anti-A as well as anti-B antibodies, group O recipients can only accept blood from group O donors. Conversely, group AB recipients can receive blood from all groups.

There are many other antigens on the red cell surface. The most important is the Rh factor. A person is defined as either Rh positive or Rh negative depending on the presence of the primary Rh antigen on the red cell. In contrast to ABO antigens, however, a person only develops anti-Rh after exposure to Rh-positive red cells through transfusion or pregnancy. Modern blood-banking technology uses highly sensitive tests to properly identify and match blood between donor and recipient.

The most common blood types in the U.S. are A positive and O positive -- about 72 percent of the population has one or the other. AB negative is the rarest blood type (1 percent of the population).

Richard J. Davey, M.D.
Chief Medical Officer
New York Blood Center

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