The ABO blood group system is the most clinically significant human blood classification, discovered by the Austrian physician Karl Landsteiner in 1901, for which he received the Nobel Prize in Physiology or Medicine in 1930. The system rests on the presence or absence of two antigens—A and B—on the surface of erythrocytes (red blood cells), and the reciprocal presence of naturally occurring antibodies (anti-A and anti-B) in the plasma. The four phenotypes are A (A antigen, anti-B antibody), B (B antigen, anti-A antibody), AB (both antigens, no antibody), and O (no antigen, both antibodies). The trait is governed by the ABO gene on chromosome 9 (locus 9q34.2), which exhibits multiple alleles (I^A, I^B, i) and codominance between I^A and I^B, while i is recessive—a standard illustration of non-Mendelian inheritance patterns in genetics syllabi.
Functionally, the ABO antigens are sugar molecules (glycoproteins/glycolipids) attached to the H antigen, a precursor substance; the A allele adds N-acetylgalactosamine, the B allele adds galactose, and the O allele produces a non-functional enzyme leaving the H antigen unmodified. This biochemistry underpins transfusion compatibility: type O is the universal donor (no A or B antigens to trigger recipient antibodies) and type AB is the universal recipient (no antibodies to attack donor cells). Mismatched transfusion causes agglutination and potentially fatal haemolytic reactions. The related but separate Rhesus (Rh) factor, also identified by Landsteiner and Wiener in 1940, determines positive or negative status and is critical in pregnancy, where Rh incompatibility can cause haemolytic disease of the newborn (erythroblastosis fetalis).
The ABO system has wide applied significance beyond medicine. In forensic science and medico-legal investigation, blood grouping historically served to exclude suspects and resolve disputed paternity before DNA fingerprinting (developed by Alec Jeffreys, 1984) became definitive. Blood group serology remains a screening tool in paternity disputes—a child's group must be consistent with parental genotypes, allowing exclusion though not positive identification. As of 2026, ABO and Rh typing remain mandatory in blood banking under national transfusion guidelines, and emerging research into enzymatic conversion of A and B blood to universal O continues to address chronic donor shortages.
For competitive examinations, the ABO system appears most directly in the General Science / Biology portions of the General Studies (Prelims) paper and in screening-test science sections across UPSC, BCS, CSS and Guokao. Typical question angles include: identifying the universal donor and recipient; matching antigens and antibodies to each blood type; recognising Karl Landsteiner and the year of discovery; explaining codominance and multiple alleles as inheritance concepts; and the role of the Rh factor in newborn haemolytic disease. Candidates should memorise the antigen-antibody table and be prepared for assertion-reasoning items on transfusion compatibility and forensic application, which are recurring high-frequency factual questions.
Example
In 1930, Karl Landsteiner received the Nobel Prize for his 1901 discovery of the ABO blood groups, which made safe blood transfusion possible and remains mandatory in every modern blood bank.
Frequently asked questions
Karl Landsteiner discovered the ABO system in 1901 and was awarded the Nobel Prize in Physiology or Medicine in 1930. He also co-identified the Rh factor with Alexander Wiener in 1940.