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Blood typing. Laboratory technician displaying blood sample typing at a blood bank, United States, 1957. Blood can be divided into four main blood groups; A, B, O and AB. A blood group is decided by the presence of antigens on the surface of red blood cells. The correct blood type must be given in transfusions to avoid an immune reaction. The major human blood groups and a technique for distinguishing between them were discovered in 1901.
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