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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 AB.)
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+ and O+ -- about 72 percent of the population has one or the other. AB- is the rarest blood type (1 percent of the population).
Photo of Karl Landsteiner: Courtesy of the National Library of Medicine.
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