The blood groups refer to the presence
on human red blood cells of certain antigens, the blood group
factors. One very important group of factors present on the red
blood cells is the ABO system. The ABO group of a person depends on
whether his/her red blood cells contain one, both, or neither of the
2 blood group antigens A and B. There are, therefore, 4 main ABO
groups: A, B, AB and O. Antibodies (agglutinins) for the antigens A
and B exist in the plasma and these are termed anti-A and anti-B.
The corresponding antigen and antibody are never found in the same
individual since, when mixed, they form antigen-antibody complexes,
effectively agglutinating the
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