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"The gift of life" (blood) is more complex than it appears to be and our in-depth understanding of the various blood groups will be instrumental in avoiding any unintended tragedy during a blood transfusion. Earlier attempts to transfer blood from one person to another produced varied results. Sometimes the person receiving the transfusion was aided by the procedure. Other times the recipient suffered a blood reaction in which the red blood cells clumped together, obstructing vessels and producing other serious consequences.

Eventually, it was discovered that each individual has a particular combination of substances in his or her blood. Some of these substances react with those in another person's blood. These discoveries led to the development of procedures for typing blood. It is now known that safe transfusions of whole blood depend upon properly matching the blood types of donors and recipients. The clumping of red blood cells following a transfusion reaction is called agglutination. This phenomenon is due to the presence of substances called agglutinogens (antigens) in the red blood cell membranes and substances called agglutinins (antibodies) dissolved in the plasma. Blood typing involves identifying the agglutinogens that are present in a person's red blood cells.Athough there are many different agglutinogens associated with human erythrocytes (red blood cells), only a few of them are likely to produce serious transfusion reactions. These include the agglutinogens of the ABO group and those of the Rh group. Avoiding the mixture of certain kinds of agglutinogens and agglutinins prevents adverse transfusion reactions.
The ABO blood group is based upon the presence (or absence) of two major agglutinogens in the red blood cell membranes - agglutinogens A and agglutinogen B-which are present at birth as a result of inheritance. The erythrocytes of each person contain one of the four following combinations of agglutinogens: only A (type A blood), only B (type B blood), both A and B (type AB blood), or neither A nor B (type O blood).Thus all humans have one of four possible blood types A, B, AB, Or O.Certain agglutinins develop spontaneously in the plasma about two to eight months after birth. Specifically, whenever agglutinogen A is absent in the red blood cells, an agglutinin called anti-A develops; and whenever agglutinogen B is absent, an agglutinin called anti-B develops. Individuals with type AB blood have neither agglutinin; and individuals with

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