The ABO system is the most important blood group antigen system.
Individuals who lack the A and/or B antigens on their red blood cells generally have
naturally occurring antibodies in their plasma that are directed against the missing
antigens. For this reason, ABO grouping should include both forward (cell) and reverse
(plasma) procedures. Proper determination of the ABO group is necessary when
selecting red blood cell-containing products for transfusion in order to prevent immediate
hemolytic transfusion reactions.