A person with AB blood carries one A allele and one B allele at the ABO gene, giving them the genotype usually written as IAIB. Both alleles are fully active: the A allele directs production of A antigens on the surface of red blood cells, and the B allele directs production of B antigens on those same cells. Because neither allele masks the other, AB is the textbook example of codominance in human genetics. The story gets more interesting once you look at subtypes, rare inheritance quirks, and what carrying both antigens means for your health.
What the A and B Alleles Actually Do
The ABO gene sits on chromosome 9 and encodes an enzyme called a glycosyltransferase. This enzyme’s job is to attach a specific sugar molecule onto a precursor structure (the H antigen) that already sits on the surface of your red blood cells. The A version of the enzyme (GTA) attaches one type of sugar, producing the A antigen. The B version (GTB) attaches a slightly different sugar, producing the B antigen. What is remarkable is how little separates the two: the A and B transferases differ at only four amino acid positions out of 354 total residues.1Journal of Biological Chemistry. ABO(H) Blood Group A and B Glycosyltransferases Recognize Substrate via Specific Conformational Changes Those four substitutions, at positions 176, 235, 266, and 268, are enough to change which sugar gets added.2Journal of Biological Chemistry. Differential Recognition of the Type I and II H Antigen Acceptors by the Human ABO(H) Blood Group A and B Glycosyltransferases
If you inherit an A allele from one parent and a B allele from the other, both enzymes are produced, and both sugars get added to red blood cells. The result is cells coated with a mixture of A and B antigens. Researchers who used fluorescent-tagged antibodies to measure antigen levels on individual red blood cells from 24 AB individuals confirmed that both antigens appear on the same cells, though the relative amounts can vary from cell to cell.3Europe PMC. Single-cell variations in the expression of codominant alleles A and B on RBC of AB blood group individuals
A person with type O blood, by contrast, has inherited two non-functional copies of the gene (typically called O alleles). The O allele produces no working transferase, so the H antigen stays unmodified. People with type A carry at least one functional A allele (genotype IAIA or IAi), and those with type B carry at least one functional B allele (IBIB or IBi). AB individuals are the only group whose genotype is unambiguous from a standard blood test: if both antigens show up, you must carry one of each allele.
Why AB Is the Rarest Common Blood Type
To be born with AB blood, you need one parent who carries an A allele and another who carries a B allele. That may sound simple, but the distribution of alleles in most populations makes this combination less common than the others. A large multicenter study across India, for instance, found that O was the most common group at about 37%, followed by B at roughly 32%, A at about 23%, and AB at just under 8%.4PubMed Central. ABO and Rh (D) group distribution and gene frequency; the first multicentric study in India The pattern varies by region, but AB is consistently the least frequent group worldwide. The reason is statistical: you need both the A and B alleles circulating in the same family, and each is individually less common than O.
Subtypes Within AB
Not all AB blood is identical. The A allele comes in subtype variants, the most common being A1 and A2. The A1 enzyme is more active, producing more A antigen per cell than the A2 enzyme does. That means a person typed as A1B has denser A antigen coating on their red cells than someone typed as A2B. The difference matters in transfusion medicine and organ transplantation, because the strength of the immune reaction to a mismatched transfusion depends partly on how much antigen is present.5PubMed. Chemical Basis for Qualitative and Quantitative Differences Between ABO Blood Groups and Subgroups: Implications for Organ Transplantation
A2B is relatively uncommon. A study of blood donors in the Jazan region of Saudi Arabia found A1B in about 1.8% of donors and A2B in roughly 0.9%, with one A2B individual producing a cold-reactive anti-A1 antibody.6PubMed Central. Prevalence of A(2) and A(2)B Subgroups and Anti-A(1) Antibody in Blood Donors in Jazan, Saudi Arabia That last detail matters: some A2B individuals make antibodies against A1 antigen, which can cause confusion during blood typing and compatibility testing. If you have ever been told your blood type gave a lab technician trouble, a weak subtype is one possible explanation.
The H Antigen Prerequisite
Before the A or B enzyme can do its work, the H antigen must already be present on the red blood cell surface. The H antigen is built by a separate enzyme encoded by the FUT1 gene. In the vast majority of people, FUT1 works normally and the H antigen is plentiful. But in rare individuals who inherit non-functional copies of FUT1, no H antigen is produced. Without that precursor, neither the A nor the B enzyme has anything to modify, and the result is a red cell with no A, B, or H antigens at all.7Journal of Men’s Health. ABO discrepancy due to the para-Bombay phenotype: a case report
This is called the Bombay phenotype (named after the Indian city where it was first identified). A person with the Bombay phenotype could carry the IAIB genotype and yet type as group O on a standard blood test, because there is simply no antigen for the typing reagents to detect. It is extremely rare, affecting roughly 1 in 10,000 people in parts of South Asia and far fewer elsewhere, but it is a genuine example of genotype and phenotype diverging. A closely related variant called para-Bombay allows partial H antigen expression, leading to weak or inconsistent typing results.
Cis-AB, the Exception That Baffles Paternity Tests
Standard genetics says the only way to get AB blood is to inherit one A allele and one B allele from separate parents. Cis-AB upends that rule. In cis-AB, a single mutant allele on one chromosome encodes an enzyme that can produce both A and B antigens. The person’s other chromosome may carry an O allele, giving them a genotype of cis-AB/O, yet they still type as AB.8PubMed Central. Cis-AB, the Blood Group of Many Faces, Is a Conundrum to the Novice Eye
The practical consequences can be startling. If a cis-AB mother has a child with a type O father, the child could inherit the cis-AB allele and type as AB, or inherit the O allele and type as O. The birth of an O child to an apparently AB mother would look like a biological impossibility under standard ABO rules and has, historically, raised false paternity concerns.9Transfusion Medicine Reviews. The cis-AB Blood Group Phenotype: Fundamental Lessons in Glycobiology Molecular sequencing of cis-AB alleles has confirmed that specific point mutations in the ABO gene alter the transferase enzyme enough to let it add both types of sugar.10PubMed. Molecular genetic analysis of the ABO blood group system: 2. cis-AB alleles
Cis-AB is uncommon but not vanishingly so. It appears most often in East Asian populations, particularly in Korea and Japan. The antigen expression is usually unbalanced, meaning one antigen (typically A) is expressed more strongly than the other. On a standard serological test the blood still reads as AB, though the weak component can occasionally cause discrepancies.
Chimerism and Mixed Genotypes
Another source of confusing ABO results is chimerism, where a single person carries cells from two genetically distinct cell lines. This can happen naturally when fraternal twins exchange blood-forming stem cells in the womb, or in rarer developmental events. A chimeric person might have some red blood cells carrying A antigen and others carrying B antigen, not because a single genome encodes both, but because two different genomes coexist in the same body.
In two well-documented cases, cloning and sequencing of the ABO gene from chimeric individuals revealed three separate alleles in their blood cells, a situation that is impossible for a single genome (which can hold at most two alleles at any locus). Short-tandem-repeat testing confirmed that one individual had received a double paternal DNA contribution, consistent with tetragametic chimerism.11Immunohematology. Chimerism and mosaicism are important causes of ABO phenotype and genotype discrepancies Even microchimerism at extremely low levels, below 1% of circulating cells, has been detected using digital PCR in cases where conventional tests missed it entirely.12PubMed. Molecular biology analysis of ABO blood group variants caused by natural chimaerism A person may test as AB serologically while actually harboring a chimeric mixture of A-only and B-only cell populations.
AB Blood in the Transfusion Setting
Because AB individuals carry both A and B antigens, their immune systems recognize both antigens as “self” and do not produce anti-A or anti-B antibodies. That means AB plasma lacks those antibodies and has traditionally been considered “universal donor” plasma, suitable for any recipient. AB red blood cells, on the other hand, can only be transfused safely to other AB recipients, since anyone with type A, B, or O blood would have antibodies against at least one of the antigens on AB cells.
The universal-plasma concept has come under some scrutiny, though. Red blood cells are not the only place ABO antigens live. Soluble A and B antigens float freely in plasma from anyone who is not type O. When AB plasma is transfused into a recipient of a different blood type, those soluble antigens can bind to the recipient’s anti-A or anti-B antibodies, forming immune complexes that may activate platelets or contribute to low-grade hemolysis.13Laboratory Medicine. Outcome of ABO-Incompatible Kidney Transplantation According to ABO Type of Transfused Plasma: Comparative Analysis Between “Universal” AB and Donor-Type Plasma The clinical significance of this is still being worked out, and in emergency settings AB plasma remains widely used for patients of unknown type. But the picture is less tidy than the classic “universal donor” label suggests.
ABO Genotyping Beyond Serology
For most day-to-day medicine, a simple serological test with anti-A and anti-B antibodies tells you everything you need. If both reagents cause clumping, the blood is AB. But as the examples of cis-AB, Bombay phenotype, weak subtypes, and chimerism show, serology can mislead. DNA-based genotyping reads the ABO gene sequence directly and can distinguish between true IAIB, cis-AB/O, Bombay with hidden A or B alleles, and chimeric mixtures.
Genotyping has proven especially useful in paternity and kinship testing. Traditional ABO phenotyping has low discriminating power because many different genotypes produce the same blood type (an A-type person could be IAIA or IAi, for example). ABO genotyping improves on this, though its power to exclude a false father is still modest compared to modern short-tandem-repeat panels. ABO blood group single-nucleotide variants could be considered as a supplementary tool alongside STR markers in future kinship analysis, partly because the ABO gene’s mutation rate is extremely low.14PubMed Central. Blood Group ABO Genotyping in Paternity Testing
Health Risks Linked to the AB Genotype
Carrying both A and B alleles is not just a transfusion detail. A growing body of research links ABO type to various disease risks, and AB sometimes stands out. One of the clearest associations is with blood clotting. ABO blood group has been recognized as a risk factor for venous blood clots since the 1960s, driven by the fact that non-O blood types are associated with higher levels of von Willebrand factor and factor VIII, two proteins that promote clotting.15PubMed Central. Is ABO blood group truly a risk factor for thrombosis and adverse outcomes? Because AB individuals carry both antigens, they tend to have some of the highest levels of these clotting factors among all blood types.
Cancer risk shows a similar pattern for at least one malignancy. A large prospective study found that compared to people with blood group O, those with AB blood had roughly a 50% higher hazard of developing pancreatic cancer (adjusted hazard ratio 1.51). Groups A and B also carried elevated risk but at somewhat lower magnitudes. Overall, the researchers estimated that about 17% of pancreatic cancer cases in the cohort were attributable to having a non-O blood group.16PubMed Central. ABO blood group and the risk of pancreatic cancer
Malaria offers another angle. A systematic review and meta-analysis confirmed that individuals with blood group A, B, or AB all had increased odds of severe malaria compared to those with group O, suggesting that the O blood group has a protective effect against the most dangerous forms of the disease.17Blood Reviews. Effect of the ABO blood group on susceptibility to severe malaria: A systematic review and meta-analysis A more granular study looking specifically at genotypes found that the AB genotype was associated with roughly 93% greater risk of severe malaria compared to OO, while BB carried about 108% greater risk.18PLOS Genetics. Non-O ABO blood group genotypes differ in their associations with Plasmodium falciparum rosetting and severe malaria The mechanism involves “rosetting,” where malaria-infected red blood cells stick to uninfected ones. A and B antigens appear to promote rosette formation more than the bare H antigen found on type O cells.
Why Evolution Has Kept All Three Alleles Around
Given that non-O blood types carry higher risks for clotting disorders and severe malaria, you might wonder why natural selection has not simply eliminated the A and B alleles. The answer is that ABO variation is extraordinarily ancient and has been actively maintained by some form of balancing selection. The A and B blood groups are not unique to humans. They appear in gibbons, Old World monkeys, and other primates, maintained as a so-called trans-species polymorphism for at least 20 million years.19PubMed Central. The ABO blood group is a trans-species polymorphism in primates Phylogenetic analysis has shown that the B allele likely arose independently from an ancestral A form at least three separate times across primate evolution, and the divergence time between human A and B alleles is far too large to be explained by random genetic drift alone.20PubMed. Evolution of primate ABO blood group genes and their homologous genes
The leading hypothesis is that co-evolution with gut pathogens, particularly bacteria and parasites that can exploit specific sugar molecules on cell surfaces, has created frequency-dependent selection pressure. When a blood type becomes common in a population, pathogens that specialize in binding its antigens gain an advantage, which then shifts selection toward the rarer types. Over millions of years, this tug-of-war keeps all three major alleles circulating.21PubMed Central. Ancestry runs deeper than blood: the evolutionary history of ABO points to cryptic variation of functional importance AB individuals, carrying both antigens, may face a distinct set of trade-offs compared to A-only or B-only carriers, which helps explain why AB exists but remains rare.
Blood Groups in Other Species
Humans are far from the only animals with blood group systems, though the details differ wildly. Domestic cattle have more than 80 recognized blood group determinants, while New World camelids (llamas, alpacas) have just one.22PubMed. Blood groups in animals Cats have an A-B blood group system that is superficially similar in name to the human one but operates on entirely different molecular machinery. The human ABO system stands out in comparative biology mainly because of its deep evolutionary roots and the strength of the selection pressures that have maintained it. The same A and B antigens you carry have functional equivalents in species separated from us by tens of millions of years of evolution, which gives some sense of how fundamental this bit of surface chemistry is to primate biology.