An acute hemolytic transfusion reaction (AHTR) is the most serious and potentially fatal complication of blood transfusion, occurring when pre-formed recipient antibodies (most commonly anti-A or anti-B isohemagglutinins) attack and rapidly destroy transfused donor red blood cells. The vast majority of AHTRs result from ABO incompatibility due to clerical or identification errors -- the wrong blood given to the wrong patient. The mortality rate ranges from 1 in 600,000 to 1 in 1,800,000 transfusions, but when AHTR occurs, the fatality rate approaches 10-44% depending on the volume transfused and the speed of recognition and intervention. The registered nurse is the final safety barrier in the transfusion chain and bears primary responsibility for verifying patient identity, confirming blood compatibility at the bedside, and recognizing the earliest signs of hemolysis. ABO blood group antigens are carbohydrate structures (oligosaccharides) expressed on the surface of red blood cells, determined by the ABO gene on chromosome 9. Type A individuals have A antigens and naturally occurring anti-B antibodies (IgM class). Type B individuals have B antigens and anti-A antibodies. Type O individuals have neither A...