Blood type

Blood is categorized into types—also called blood groups—based on whether certain inherited markers (antigens) sit on the surface of red blood cells, and whether corresponding antibodies are present. These antigens can be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the system they belong to.

A person inherits their blood type from both parents. As of June 2025, the International Society of Blood Transfusion recognizes 48 distinct human blood group systems. The ABO and Rh systems are the most critical for blood transfusions; together, they define a person’s type as A, B, AB, or O, with a plus or minus sign indicating the presence or absence of the RhD antigen.

Blood type systems

A full blood type would account for all 48 groups, and an individual’s type is one of countless possible antigen combinations. In nearly all cases, a person’s blood type stays the same for life. Rarely, an infection, malignancy, or autoimmune disease can add or suppress an antigen, changing the type.

A more common cause of change is a bone marrow transplant, often used for leukemias and lymphomas. If a recipient’s marrow is replaced by donor marrow of a different ABO type—say, a type O patient receives type A marrow—the patient’s blood type eventually shifts to the donor’s type. This happens because the patient’s own hematopoietic stem cells are destroyed (by marrow ablation or donor T-cells), and once all original red blood cells die off, new cells from the donor stem cells take over, carrying different surface antigens.

Some blood types are linked to inherited diseases; for instance, the Kell antigen can be tied to McLeod syndrome. Certain types also affect infection susceptibility—people lacking the Duffy antigen, for example, show resistance to specific malaria species. The Duffy antigen is less common in populations from malaria-heavy regions, likely due to natural selection.

ABO blood group system

The ABO system revolves around two antigens (A and B) and two antibodies (anti-A and anti-B). Antigens sit on red blood cells; antibodies float in the serum. This yields four groups: those with only antigen A (group A), only antigen B (group B), both A and B (group AB), or neither (group O).

Antibodies pair accordingly: group A has anti-B, group B has anti-A, group AB has neither, and group O has both. When matching antigen and antibody meet, they agglutinate (clump). So a transfusion is safe if the recipient’s serum lacks antibodies against the donor’s red cell antigens.

The ABO system is the most important for transfusion. Its anti-A and anti-B antibodies are typically IgM, thought to arise in early childhood from exposure to environmental substances like food, bacteria, and viruses. Karl Landsteiner originally labeled the groups A, B, and C in 1901; later, “C” became “O.” In many languages, type O is called 0 (zero or null).

Rh blood group system

The Rh system is the second most significant for transfusion, with 50 antigens currently known. The D antigen is the most important because it most often triggers an immune response. D-negative individuals usually lack anti-D antibodies (IgG or IgM), since these aren’t typically produced by environmental exposure.

However, a sensitizing event—like a fetomaternal transfusion during pregnancy or a transfusion with D-positive red cells—can cause D-negative people to generate IgG anti-D antibodies. Rh-negative types are far rarer in Asian populations (0.3%) than in European ones (15%). The plus or minus sign after an ABO type indicates Rh(D) status; for example, A− means type A without the Rh(D) antigen.

ABO and Rh distribution by country

Like many genetic traits, ABO and Rh distributions vary widely by population. Scientists debate why blood types differ geographically and why they emerged, but evidence suggests evolution may have favored types whose antigens protect against local diseases. For instance, type O is common in malaria-endemic regions, where people with type O show the highest survival rates.

As of June 2025, 48 blood-group systems have been identified and recognized by the International Society for Blood Transfusion.