ABO blood group system
Last updated
The ABO blood group system is used to denote the presence of one, both, or neither of the A and B antigens on erythrocytes (red blood cells). For human blood transfusions, it is the most important of the 48 different blood type classification systems currently recognized by the International Society of Blood Transfusions as of June 2025. A mismatch in this serotype can cause a potentially fatal adverse reaction after a transfusion or an unwanted immune response to an organ transplant, though such mismatches are rare in modern medicine.
The associated anti-A and anti-B antibodies are usually IgM antibodies, produced in the first years of life by sensitization to environmental substances such as food, bacteria, and viruses. The ABO blood types were discovered by Karl Landsteiner in 1901, for which he received the Nobel Prize in Physiology or Medicine in 1930. Landsteiner found that red blood cells would clump together when mixed with sera from different persons, and he classified human blood into three groups: A, B, and C. Group C, later designated as O, indicated the absence of both A and B antigens but contained anti-A and anti-B antibodies. The fourth type, AB, was discovered the following year by his students Adriano Sturli and Alfred von Decastello.
In 1910, Ludwik Hirszfeld and Emil Freiherr von Dungern introduced the terms O and AB and explained the genetic inheritance of the blood groups. Confusion arose from competing numerical classification systems by Jan Janský and William L. Moss, which was resolved in 1927 when Landsteiner proposed the letter-based system O, A, B, and AB, later universally adopted. ABO blood types are also present in other primates such as apes, monkeys, and Old World monkeys.
Quick Facts
- Discovered by
- Karl Landsteiner
- Key antigens
- A and B
- Key antibodies
- Anti-A and Anti-B (usually IgM)
- Gene location
- Chromosome 9 (9q34)
- Inheritance
- Multiple alleles (IA, IB, i) with codominance
Facts from the source article.
Lore & Background
The ABO blood group system classifies blood based on the presence or absence of A and B antigens on the surface of red blood cells. Individuals may have one, both, or neither of these antigens. The corresponding antibodies, primarily of the IgM type, are typically present in the serum: anti-A and anti-B. These antibodies are not present at birth but develop during the first years of life, triggered by exposure to environmental substances such as food, bacteria, and viruses. The system is the most clinically significant of the 48 blood group classification systems recognized by the International Society of Blood Transfusions, as a mismatch in ABO type can provoke a potentially fatal immune reaction during transfusion or organ transplantation, though such mismatches are rare in modern practice.
ABO blood types are not unique to humans; they are also found in other primates, including apes, monkeys, and Old World monkeys. The antigens are composed of specific sugars: N-acetylgalactosamine confers the A-type, while galactose confers the B-type. These ABH antigenic substances are carried on glycoproteins within the red blood cell membrane, predominantly on Band 3 and Band 4.5 proteins, and are attached to polylactosamine chains.
Reader's Guide
The ABO blood group system is foundational to transfusion medicine. A mismatch can cause a potentially fatal adverse reaction, though such mismatches are rare in modern practice. Early classification confusion arose from competing numerical systems by Jan Janský and William L. The ABO gene on chromosome 9 encodes a glycosyltransferase that modifies red cell antigens.
The rare Bombay phenotype (hh) produces antibodies against A, B, and O groups, limiting transfusions to other hh individuals. Subgroups exist, notably A1 and A2, which are not always interchangeable. ABO blood types are also present in other primates.
Did You Know?
- The ABO system is the most important of 43 blood type systems recognized by the ISBT as of the last widely known update in 2023.
- The cis-AB phenotype has a single enzyme that creates both A and B antigens.
Frequently Asked Questions
What is ABO blood group system known for?
It dictates whether red blood cells display A antigens, B antigens, both, or neither, and it determines which corresponding antibodies (Anti-A and Anti-B, typically IgM) circulate in the plasma. In practice, this makes it the primary compatibility gatekeeper for safe blood transfusions and organ transplants.
What is ABO blood group system's origin story (inheritance pattern)?
The system is encoded on chromosome 9 (region 9q34) and follows a multiple-allele model involving IA, IB, and i, with A and B acting as codominant over the recessive i allele. This is why a child can end up with a blood type that neither parent carries.
More in Cardiovascular & Blood
Sources
Compiled from Wikipedia and the sources listed below. Text from Wikipedia is available under CC BY-SA 4.0; this entry is adapted from it.
- Wikipedia: ABO blood group system (CC BY-SA 4.0).
- Word definitions: the Codexery glossary, each quoted from its Wikipedia article.
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced