The ABO typing is the most important test performed in transfusion practice today. The most common cause of life-threatening hemolytic transfusion reactions is due to a patient being transfused with ABO incompatible blood. These reactions occur because individuals form potent, naturally occurring antibodies to ABO red cell antigens, which they do not possess. These antibodies have the ability to fix complement and cause intravascular hemolysis of incompatible RBCs. When transfused with ABO incompatible blood, an immediate antigen/antibody reaction occurs which, if not detected in time, may be fatal. Because of the dire consequences associated with ABO incompatible transfusions, ABO typing and compatibility testing remain the foundation of all pre-transfusion testing.
The terms D positive and D negative refer to the presence or absence of the D antigen on the RBC. Approximately 85% of the general population has the D antigen on their RBCs. After A and B antigens, the D antigen is the most important antigen in transfusion practice. The D antigen is highly immunogenic. Individuals, who lack the D antigen, may form potent immune antibodies to the D antigen. These individuals must be given D negative blood to prevent antibody stimulation.