- The ABO blood group system is the most important and widely recognized method of classifying human blood based on the presence or absence of two antigens—A and B—on the surface of red blood cells (RBCs). First discovered by Karl Landsteiner in 1901, this system revolutionized medicine by enabling safe blood transfusions and has since become a fundamental concept in immunohematology, transfusion science, and clinical medicine.
- In the ABO system, there are four major blood groups:
- Group A individuals have A antigens on the surface of their RBCs and produce anti-B antibodies in their plasma.
- Group B individuals have B antigens on their RBCs and produce anti-A antibodies.
- Group AB individuals have both A and B antigens on their red cells and produce no ABO antibodies in the plasma. This makes them universal recipients for red cell transfusions within the ABO system.
- Group O individuals have neither A nor B antigens on their RBCs but produce both anti-A and anti-B antibodies. This makes them universal donors for red cells, as their blood is least likely to cause a reaction in recipients of other blood types.
- The antibodies (anti-A and anti-B) found in the plasma are naturally occurring, meaning individuals do not need prior exposure to foreign blood to develop them. This is unlike the antibodies in the Rh system, which require sensitization. The ABO antibodies are IgM class, which are potent and can cause serious reactions if mismatched blood is transfused.
- The implications of this system are most evident in blood transfusions. If a person receives blood that carries antigens incompatible with their own, their immune system will recognize the foreign antigens as threats and trigger a hemolytic transfusion reaction. This can lead to symptoms such as fever, chills, hemoglobinuria, and in severe cases, kidney failure or death. Therefore, blood type matching is a critical safety step before transfusion.
- From a genetic standpoint, the ABO blood type is determined by the ABO gene on chromosome 9, which has three main alleles: A, B, and O. The A and B alleles are co-dominant, meaning both can be expressed when present together (resulting in type AB). The O allele is recessive, so an individual needs two copies of it to be type O.
- The distribution of ABO blood types varies among different populations and ethnic groups. For instance, type O is the most common worldwide, especially in Latin American populations, while type B is more prevalent in parts of Asia. These differences can have implications for blood supply and demand in different regions.
