How does the ABO blood group system classify blood?

The ABO blood group system classifies blood primarily by identifying the presence or absence of specific A and B antigens on the surface of red blood cells. Individuals who possess the A antigen are classified as type A, those with the B antigen as type B, those with both antigens as type AB, and those with neither antigen as type O. This classification is crucial for blood transfusions, organ transplants, and understanding immune responses, as the presence of incompatible blood group antigens can lead to severe reactions.

Other classifications, such as levels of hemoglobin, white blood cell count, or blood clotting factors, do not pertain to the ABO blood group determination and are used for different medical evaluations. Hemoglobin levels relate to anemia or oxygen-carrying capacity, white blood cell counts are indicators of immune status or infection, and clotting factors are assessed in cases of bleeding disorders. These elements do not influence the ABO classification, making the identification of antigen presence the definitive method for blood group classification.