ABO Blood Types: How Antigens Determine Compatibility

Human ABO blood types classify blood based on inherited carbohydrate antigens on red blood cells and matching antibodies in blood serum. This framework divides human blood into four main groups named A, B, AB, and O. Because antibodies attack foreign antigens during a transfusion, typing these cell markers is the key step to prevent fatal immune reactions during surgery [7].

Antigen Architecture on Red Cells

Red blood cell membranes display branching sugar chains that establish blood groups. These surface structures are assembled on top of a shared core called the H substance. Specific glycosyltransferase enzymes attach sugar molecules to the end of this base chain. In group A people, the enzyme adds N-acetylgalactosamine (GalNAc) to complete the A antigen. In group B people, a related transferase attaches an extra D-galactose molecule to form the B antigen [10].

In type O people, an inactive protein leaves the basic H structure unchanged. Red cells in group O carry only this bare foundation on their outer lipid wall. Human red cells anchor these sugars inside large glycoproteins like Band 3, Band 4.5, and glycophorin complexes [6].

Chemical analysis reveals that the A antigen forms a terminal trisaccharide consisting of N-acetylgalactosamine bound to fucose and galactose, but the B antigen features galactose bound to the same core structure. The unchanged H antigen remains a simpler disaccharide terminating in fucose and galactose. Because these sugar chains project into plasma, antibodies bind their shapes quickly. Watkins and Morgan in England discovered that these ABO epitopes are conferred by sugars, and Finnish researcher Jukka Finne showed in 1978 that red cell glycoproteins carry polylactosamine chains bearing ABH substances. Surface sugar architecture serves as the physical trigger for immune compatibility [6, 10].

How ABO Blood Types Function in Transfusions

Transfusion safety relies on matching donor red cell antigens against recipient plasma antibodies to avoid cell destruction. People with type A blood produce anti-B antibodies, but people with type B blood carry anti-A antibodies. Group O people produce both anti-A and anti-B antibodies in their serum. In contrast, group AB people produce neither antibody, so their plasma does not attack donor red blood cells from any of the four ABO blood types [2].

Naturally occurring defenses against foreign blood belong primarily to the IgM class of antibodies and develop during early infancy through exposure to environmental substances found in food, bacteria, and viruses. When incompatible donor blood enters a patient’s veins, recipient IgM antibodies bind to the foreign red blood cells and activate complement proteins, which triggers rapid intravascular hemolysis. This destruction releases free hemoglobin into plasma, causing acute kidney injury, shock, and circulatory collapse if doctors do not halt the infusion immediately. Safe transfusions require that donor red cells lack antigens that match recipient antibodies [7].

Hospital blood banks enforce infusion limits to detect adverse reactions early. Transfusion guidance from the Department of Pathology at the University of Michigan mandates a first red cell infusion rate of no more than 15 milliliters in the first 15 minutes. Standard transfusions take two hours and must finish within four hours [2].

ABO Blood Types and Gene Inheritance

Human ABO blood types follow Mendelian inheritance patterns controlled by a single gene located on the long arm of chromosome 9 at position 9q34. Classical genetics identifies three main alleles at this locus, designated IA, IB, and i, which encode the glycosyltransferase variants that build the antigens. Alleles IA and IB show codominance toward each other, meaning both express their respective sugars when inherited together, but both remain fully dominant over the recessive i allele. German mathematician Felix Bernstein demonstrated this single locus model with multiple alleles in 1924, replacing earlier dual-gene theories. This genetic arrangement establishes the AB phenotype with full expression of both antigens [6].

Six possible genotypes produce the four visible blood groups because each person inherits one allele from each parent. Genotypes IAIA and IAi yield type A blood, but IBIB and IBi generate type B blood. Only the homozygous ii genotype produces type O blood. When parents who both carry heterozygous genes such as IAi and IBi have children, each child has a 25 percent chance of inheriting type A, type B, type AB, or type O blood [6].

In 1990, a research team led by Yamamoto cloned the complementary DNAs for human A, B, and O alleles at the ABO locus. Their work showed that four amino acid substitutions distinguish A and B transferases. Furthermore, the common O allele carries a single guanosine nucleotide deletion at position 261. This frame-shift mutation yields a truncated protein that cannot modify the H antigen [9].

Discovery by Karl Landsteiner

Austrian physician Karl Landsteiner discovered the blood grouping system in 1900 while working at the Pathological-Anatomical Institute of the University of Vienna. During his laboratory experiments, Landsteiner mixed blood serum from coworkers with red cells from other staff members, noticing that some combinations clumped together while others stayed liquid. He recorded this observation in a footnote, stating that serum from healthy people agglutinates red cells from other humans in a pattern suggesting inborn biological differences [6].

In 1901, Landsteiner published his grouping system dividing human blood into three categories labeled A, B, and C, with group C later called type O. The following year, his assistants Adriano Sturli and Alfred von Decastello discovered the fourth group, which clumped with both anti-A and anti-B sera. Ludwik Hirszfeld and Emil Freiherr von Dungern introduced the terms O and AB in 1910, proving that blood groups pass to offspring by regular inheritance [6].

Clinical use expanded after American physician Reuben Ottenberg performed the first transfusion with blood typing in 1907. Large-scale transfusions grew during the First World War when doctors began adding citric acid between 1914 and 1915 to stop clotting in storage. Conflicting Roman numeral systems devised by Czech serologist Jan Janský in 1907 and American physician William L. Moss in Britain and France caused severe hospital confusion because they reversed numbering. Landsteiner moved to the Rockefeller Institute for Medical Research in New York and recommended adopting the letters O, A, B, and AB through the National Research Council in 1927. He received the Nobel Prize in Physiology or Medicine in 1930 for his discovery [6].

Subgroups Beyond the Four Main Types

The main blood groups contain internal variations that can complicate laboratory tests. Type A blood includes twenty recognized subgroups, dominated by A1 and A2, which account for over 99 percent of type A people. The A1 subgroup represents about 80 percent of this group, but A2 makes up almost all the rest. Because A2 cells carry fewer antigen sites, some A2 patients produce anti-A1 antibodies that need extra cross-matching before transfusion [6].

Rare genetic variants occasionally produce unexpected inheritance patterns. People with the rare Bombay phenotype, denoted genetically as hh, cannot synthesize the foundational H substance and produce strong antibodies against A, B, and O blood. Another variant known as cis-AB arises when one parent transmits both A and B transferase activities on a single chromosome. Under this rare transmission, an AB parent and an O parent can have an O child, which was once thought genetically impossible [6].

Medical treatments can alter a person’s blood group under specific conditions. In bone marrow transplants for leukemia or lymphoma, clinicians ablate host marrow before infusing donor hematopoietic stem cells. As donor stem cells take over red cell production, the patient’s circulatory system gradually converts to the donor’s blood type [7].

ABO Blood Types Across World Populations

Frequencies of ABO blood types differ substantially across geographic regions and ethnic groups. Group O is the most common phenotype worldwide, exceeding 70 percent among indigenous groups in the Americas and parts of Central Africa. Group A occurs with high frequency in Scandinavia and Central Europe, but group B reaches its highest share in Central Asia and the Middle East, while remaining absent from Native American and Australian Aboriginal populations prior to European contact [8].

Regional donor records illustrate these geographic variations with high laboratory precision. In a study published in the Sudan Journal of Medical Sciences, Christ-Dominique Ngassaki-Yoka, Jophrette Mireille Ntsame Ndong, Cyrille Bisseye, and Malavika Suresh tested 4,744 blood donors in Libreville, Gabon. Using DIAGAST QWALYS 3 automated testing and BIO-RAD ID Card gel cards, they found phenotypic rates of 58.9 percent for group O, 21.0 percent for group A, 17.6 percent for group B, and 2.6 percent for group AB. Their analysis showed that 97.7 percent of Gabonese donors were Rhesus D positive, contrasting with European populations where Rh-negative individuals comprise about 15 percent [5].

Other African donor surveys show comparable patterns with local differences. At the Bahir Dar blood bank in Ethiopia, a retrospective study of 27,027 blood donors by Legese and colleagues found that group O was the most common type at 41.4 percent. In Nigeria, a study of 2,388 donors at four regional blood facilities in Ado, Ikole, Ijero, and Ikere by Anyiam and coworkers recorded group O at 78.2 percent, but group AB at only 0.9 percent [13, 14].

Disease Susceptibility and Natural Selection

Epidemiological research connects ABO blood types to risks for chronic and infectious illnesses. In a meta-analysis of 1,045 gastric cancer patients from Ruijin Hospital in Shanghai and 53,026 healthy controls, Wang and colleagues showed that group A carried a higher risk of gastric cancer, with an odds ratio of 1.34 compared to non-A groups. Group A people also showed higher rates of Helicobacter pylori infection, but group O individuals showed a reduced cancer risk. Harvey G. Klein from the National Institutes of Health noted that group O people face higher risks of peptic ulcers instead [4, 11].

Cardiovascular studies reveal similar patterns across blood groups. A meta-analysis of 17 studies covering 225,810 people by Chen and colleagues showed that blood group A carried higher risk for coronary artery disease, but group O lowered risk. Infectious diseases also show strong links: the Gabon donor study by Ngassaki-Yoka found that hepatitis B virus seroprevalence was twice as high in non-O blood groups compared to group O donors [5, 12].

Balancing selection driven by pathogens likely maintained these blood groups over evolutionary timescales. In a 2012 research preprint on arXiv, Laure Ségurel, Molly Przeworski, and coworkers compared humans, gibbons, and Old World monkeys, concluding that ABO variation is an ancient trans-species polymorphism maintained for tens of millions of years outside the Major Histocompatibility Complex. Because this paper is an unreviewed preprint, researchers view its modeling as provisional until confirmed by peer-reviewed studies. Klein observed that group O people are more vulnerable to plague, but group A people are more susceptible to smallpox [1, 4].

Compatibility Protocols in Modern Medicine

Today’s blood banking separates whole blood into components, applying distinct matching rules for red cells and plasma. Type O people serve as universal red cell donors because their erythrocytes lack A and B antigens. Conversely, type AB people serve as universal plasma donors because their plasma lacks anti-A and anti-B antibodies, which allows safe delivery to patients of any blood group [2].

Blood banks manage platelets and cryoprecipitate under practical clinical rules. Transfusion guidelines from the Department of Pathology at the University of Michigan note that while identical ABO matching is the first choice, patients can receive platelets or cryoprecipitate of any ABO type when stocks are low. In addition, Rhesus matching does not apply to plasma or cryoprecipitate components, helping emergency trauma teams save time [2].

While ABO and Rhesus systems govern daily hospital care, hematologists continue to discover new blood group antigens. In June 2025, the International Society of Blood Transfusion recognized the 48th system after Thierry Peyrard at the French Blood Establishment identified the Gwada negative type in a 54-year-old woman from Guadeloupe living in Paris. By September 2026, the ISBT catalog grew to 49 recognized systems, showing that high-throughput DNA sequencing continues to improve transfusion care [3, 7].

1. ^PREPRINT Ségurel, L., Thompson, E. E., Flutre, T., Lovstad, J., Venkat, A., Margulis, S. W., Moyse, J., Ross, S., Gamble, K., Sella, G., Ober, C., & Przeworski, M. (2012). Blood ties: ABO is a trans-species polymorphism in primates. arXiv. [Article Link]
2. ^WEBSITE University of Michigan Department of Pathology. (2013). Blood component ABO compatibility chart and transfusion guidelines. Inside Pathology. [Article Link]
3. ^{ONLINE NEWS} CBS News. (2025). French scientists discover new blood type in Guadeloupe woman. CBS News. [Article Link]
4. ^{ONLINE NEWS} Klein, H. G. (2005). Why do people have different blood types? Scientific American. [Article Link]
5. ^{ACADEMIC JOURNAL} Ngassaki-Yoka, C. D., Ntsame Ndong, J. M., Bisseye, C., & Suresh, M. (2018). ABO, Rhesus blood groups and transfusion-transmitted infections among blood donors in Gabon. Sudan Journal of Medical Sciences, 13(1), 12–21. [Article Link]
6. ^WEBSITE Wikipedia. (2026). ABO blood group system. In Wikipedia. [Article Link]
7. ^WEBSITE Wikipedia. (2026). Blood type. In Wikipedia. [Article Link]
8. ^WEBSITE Wikipedia. (2026). Blood type distribution by country. In Wikipedia. [Article Link]
9. ^{ACADEMIC JOURNAL} Yamamoto, F. (2021). Molecular genetics and genomics of the ABO blood group system. Annals of Blood, 6, 25. [Article Link]
10. ^{ACADEMIC JOURNAL} Misevic, G. N. (2018). ABO blood group system. Blood and Genomics, 2(2), 71–84. [Article Link]
11. ^{ACADEMIC JOURNAL} Wang, Z., Liu, L., Ji, J. H., Zhang, J., Yan, M., Zhang, J., Liu, B., Zhu, Z., & Yu, Y. (2012). ABO blood group system and gastric cancer: A case-control study and meta-analysis. International Journal of Molecular Sciences, 13(10), 13308–13321. [Article Link]
12. ^{ACADEMIC JOURNAL} Chen, Z., Yang, S., Xu, H., & Li, J. (2016). ABO blood group system and the coronary artery disease: An updated systematic review and meta-analysis. Scientific Reports, 6(1), 23250. [Article Link]
13. ^{ACADEMIC JOURNAL} Legese, B., Shiferaw, M., Tamir, W., Eyayu, T., Damtie, S., Berhan, A., Getie, B., Abebaw, A., & Solomon, Y. (2022). Association of ABO and Rhesus blood types with transfusion-transmitted infections among apparently healthy blood donors at Bahir Dar blood bank, Bahir Dar, North West, Ethiopia: A retrospective cross-sectional study. Journal of Blood Medicine, 13, 581–587. [Article Link]
14. ^{ACADEMIC JOURNAL} Anyiam, A. F., Arinze-Anyiam, O. C., Irondi, E. A., & Obeagu, E. I. (2023). Distribution of ABO and rhesus blood grouping with HIV infection among blood donors in Ekiti State Nigeria. Medicine, 102(47), e36342. [Article Link]

Cover image: Arek Socha / Wikimedia Commons, CC0