The ABO system is the most significant method for classifying human blood, based on inherited characteristics found on the surface of red blood cells. This classification is foundational in medicine, particularly for ensuring safe blood transfusions. A mismatch between donor and recipient blood types can trigger a severe, life-threatening immune response.
Knowing an individual’s ABO type is the first necessary step before any medical procedure involving blood or organ transplants. The system divides blood into four main groups—A, B, AB, and O—which determine compatibility. The ABO blood group system was discovered in 1901 by Karl Landsteiner, a finding that later earned him the Nobel Prize.
The Underlying Biology of Blood Types
Blood type classification is determined by the presence or absence of specific markers, called antigens, which are found on the surface of red blood cells (RBCs). The ABO system involves two main carbohydrate antigens, designated A and B. The presence of these antigens dictates an individual’s blood type: Type A blood has the A antigen, Type B has the B antigen, and Type AB has both. The absence of both the A and B antigens results in Type O blood.
The liquid portion of the blood, called plasma, contains corresponding proteins known as antibodies. These antibodies are naturally occurring components of the immune system that recognize and attack foreign antigens. A person develops antibodies against the antigens their own red blood cells do not carry. For example, a person with Type A blood naturally produces anti-B antibodies, while a person with Type O blood produces both anti-A and anti-B antibodies. The existence of these pre-formed antibodies is why the ABO system is so important for transfusions, as a reaction can occur immediately upon mixing incompatible blood.
The Genetics of Blood Type Inheritance
The ABO blood type is determined by a single gene located on chromosome 9, which is inherited from both parents. This gene has three common alternative forms, known as alleles: $I^A$, $I^B$, and $i$. An individual inherits one allele from each biological parent, resulting in six possible combinations, or genotypes.
The $I^A$ and $I^B$ alleles are dominant over the $i$ allele, which is recessive. This means the genotypes $I^A I^A$ and $I^A i$ both result in Type A blood, while $I^B I^B$ and $I^B i$ result in Type B blood. The $i$ allele is recessive to both A and B, and the genotype $i i$ results in Type O blood.
The A and B alleles are considered codominant. When a person inherits both the $I^A$ and $I^B$ alleles, the resulting genotype $I^A I^B$ causes both the A and B antigens to be expressed simultaneously on the red blood cell surface, resulting in Type AB blood. This system of dominance and codominance explains how a child’s blood type may differ from either parent’s, as the combination of two alleles determines the final blood group.
Understanding Blood Transfusion Compatibility
The purpose of classifying blood types is to prevent a dangerous immune reaction during a blood transfusion. If a recipient receives incompatible blood, their pre-formed antibodies will bind to the foreign antigens on the donor’s red blood cells, causing them to clump together—a reaction known as agglutination.
Agglutination can block small blood vessels and cause the donated red blood cells to rupture, releasing toxic hemoglobin into the bloodstream. This acute hemolytic transfusion reaction can rapidly lead to serious complications such as fever, kidney failure, and death. Compatibility testing is a fundamental safety check before any transfusion.
Type O red blood cells are the “universal donor” because they lack both A and B antigens. Conversely, Type AB blood is the “universal recipient” for red blood cells because individuals with this type possess both A and B antigens, and therefore produce neither anti-A nor anti-B antibodies. While Type O blood is universally compatible for red blood cell donation, Type O individuals can only receive Type O blood themselves. In emergency situations where there is no time to type the patient’s blood, O-negative blood is often administered as a default, as it is the safest option for nearly all patients.
The Critical Role of the Rhesus Factor
In addition to the ABO classification, blood is categorized by the Rhesus (Rh) factor, which is a separate blood group system. The Rh factor refers to the presence or absence of the D antigen on the surface of red blood cells. If the D antigen is present, the blood is considered Rh-positive, and if it is absent, the blood is Rh-negative. Roughly 85% of people are Rh-positive, and the positive or negative sign after the ABO type indicates this status (e.g., A+ or O-).
The Rh factor is important for transfusions and particularly during pregnancy. An Rh-negative person receiving Rh-positive blood can develop anti-D antibodies, causing a reaction if they receive Rh-positive blood again. The Rh factor is also a major concern when an Rh-negative mother carries an Rh-positive fetus. If the baby’s blood enters the mother’s bloodstream, her immune system produces anti-D antibodies. These antibodies can cross the placenta in a subsequent pregnancy, potentially attacking the red blood cells of a future Rh-positive baby and causing Rh disease.