A blood type (also known as a blood group) is a classification of blood based on the presence and absence of antibodies and inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be proteins, carbohydrates, glycoproteins, or glycolipids, depending on the blood group. Some of these antigens are also present on the surface of other types of cells of various tissues. Several of these red blood cell surface antigens can stem from one allele (or an alternative version of a gene) and collectively form a blood group system.
Blood types are inherited and represent contributions from both parents of an individual. As of September 2026, a total of 49 human blood group systems are recognized by the International Society of Blood Transfusion (ISBT). The two most important blood group systems are ABO and Rh; they determine someone's blood type (A, B, AB, and O, with + or − denoting RhD status) for suitability in blood transfusion.
Blood type systems
A complete blood type would describe each of the 49 blood groups, and an individual's blood type is one of many possible combinations of blood-group antigens. Almost always, an individual has the same blood group for life, but very rarely an individual's blood type changes through addition or suppression of an antigen in infection, malignancy, or autoimmune disease. Another more common cause of blood type change is a bone marrow transplant. Bone-marrow transplants are performed for many leukemias and lymphomas, among other diseases. If a person receives bone marrow from someone of a different ABO type (e.g., a type O patient receives a type A bone marrow), the patient's blood type should eventually become the donor's type, as the patient's hematopoietic stem cells (HSCs) are destroyed, either by ablation of the bone marrow or by the donor's T-cells. Once all the patient's original red blood cells have died, they will have been fully replaced by new cells derived from the donor HSCs. Provided the donor had a different ABO type, the new cells' surface antigens will be different from those on the surface of the patient's original red blood cells.
Some blood types are associated with the inheritance of other diseases; for example, the Kell antigen is sometimes associated with McLeod syndrome. Certain blood types may affect susceptibility to infections, such as the resistance to specific malaria species seen in individuals lacking the Duffy antigen. The Duffy antigen, presumably as a result of natural selection, is less common in population groups from areas having a high incidence of malaria.
ABO blood group system
The ABO blood group system involves two antigens and two antibodies found in human blood. The two antigens are antigen A and antigen B. The two antibodies are A and B. The antigens are present on the red blood cells, and the antibodies in the serum. Regarding the antigen property of the blood, all human beings can be classified into four groups: those with antigen A (group A), those with antigen B (group B), those with both antigen A and B (group AB), and those with neither antigen (group O). The antibodies present together with the antigens are found as follows: Antigen A with antibody B Antigen B with antibody A Antigen AB with neither antibody A nor B Antigen null (group O) with both antibodies A and B
There is an agglutination reaction between similar antigen and antibody (for example, antigen A agglutinates the antibody A, and antigen B agglutinates the antibody B). Thus, transfusion can be considered safe as long as the serum of the recipient does not contain antibodies for the blood cell antigens of the donor.
The ABO system is the most important blood-group system in human-blood transfusion. The associated anti-A and anti-B antibodies are usually immunoglobulin M, abbreviated IgM, antibodies. It has been hypothesized that ABO IgM antibodies are produced in the first years of life by sensitization to environmental substances such as food, bacteria, and viruses. The original terminology used by Karl Landsteiner in 1901 for the classification was A/B/C; in later publications "C" became "O". Type O is often called 0 (zero, or null) in other languages. +Phenotype and genotype of blood typesPhenotypeAlleles A ABO*A1.01 B ABO*B1.01 AB ABO*A1.01, ABO*B1.01 O Two nonfunctional ABO genes
Rh blood group system
The Rh system (Rh meaning Rhesus) is the second most significant blood-group system in human blood transfusion, with currently 50 antigens. The most significant Rh antigen is the D antigen, because it is the most likely to provoke an immune system response of the five main Rh antigens. It is common for D-negative individuals not to have any anti-D IgG or IgM antibodies, because anti-D antibodies are not usually produced by sensitization against environmental substances. However, D-negative individuals can produce IgG anti-D antibodies following a sensitizing event: possibly a fetomaternal transfusion of blood from a fetus in pregnancy or occasionally a blood transfusion with D-positive RBCs. Rh negative blood types are much less common in Asian populations (0.3%) than they are in European populations (15%).
The presence or absence of the Rh(D) antigen is signified by the + or − sign, so that, for example, the A− group is ABO type A and does not have the Rh (D) antigen.
ABO and Rh distribution by country
As with many other genetic traits, the distribution of ABO and Rh blood groups varies significantly between populations. While theories are still debated in the scientific community as to why blood types vary geographically and why they emerged in the first place, evidence suggests that the evolution of blood types may be driven by genetic selection for those types whose antigens confer resistance to particular diseases in certain regions – such as the prevalence of blood type O in malaria-endemic countries where individuals of blood type O exhibit the highest rates of survival.
Other blood group systems
As of September 2026, 49 blood-group systems have been identified and are recognized by the International Society for Blood Transfusion. Thus, in addition to the ABO antigens and Rh antigens, many other antigens are expressed on the RBC surface membrane. For example, an individual can be AB, D positive, and at the same time M and N positive (MNS system), K positive (Kell system), Lea or Leb negative (Lewis system). Many of the blood group systems were named after the patients in whom the corresponding antibodies were initially encountered. Blood group systems other than ABO and Rh pose a potential, yet relatively low, risk of complications upon mixing of blood from different people.
Following is a comparison of clinically relevant characteristics of antibodies against the main human blood group systems: ABO Rh Kell Duffy Kidd Naturally occurring Yes No No No No Most common in immediate hemolytic transfusion reactions A Yes Fya Jka Most common in delayed hemolytic transfusion reactions E, D, C Jka Most common in hemolytic disease of the newborn Yes D, C Yes Commonly produce intravascular hemolysis Yes Yes
Clinical significance
Blood transfusion
Transfusion medicine is a specialized branch of hematology that is concerned with the study of blood groups, along with the work of a blood bank that provides a transfusion service with blood and other blood products. Across the world, blood products must be prescribed by a medical doctor (licensed physician or surgeon) in a similar way as medicines.
Much of the routine work of a blood bank involves testing blood from both donors and recipients to ensure that every individual recipient is given blood that is compatible and as safe as possible. If a unit of incompatible blood is transfused between a donor and recipient, a severe acute hemolytic reaction with hemolysis (RBC destruction), kidney failure and shock is likely to occur, and death is a possibility. Antibodies can be highly active and can attack RBCs and bind components of the complement system to cause massive hemolysis of the transfused blood.
Patients should ideally receive their own blood or type-specific blood products to minimize the chance of a transfusion reaction. It is also possible to use the patient's own blood for transfusion. This is called autologous blood transfusion, which is always compatible with the patient. The procedure of washing a patient's own red blood cells goes as follows: The patient's lost blood is collected and washed with a saline solution. The washing procedure yields concentrated washed red blood cells. The last step is reinfusing the packed red blood cells into the patient. There are multiple ways to wash red blood cells. The two main ways are centrifugation and filtration methods. This procedure can be performed with microfiltration devices. Risks can be further reduced by cross-matching blood, but this may be skipped when blood is required for an emergency. The oldest form of cross-matching involves mixing a sample of the recipient's serum with a sample of the donor's red blood cells and checking if the mixture agglutinates or forms clumps. If agglutination is not obvious by direct vision, a blood bank technologist may check for agglutination with a microscope. If agglutination occurs, that donor's blood cannot be transfused to that particular recipient. In a bank transfusion service, all blood specimens must be correctly identified, so labelling has been standardized using a barcode system known as ISBT 128.
The blood group may be included on identification tags or historically on tattoos worn by military personnel, in case they should need an emergency blood transfusion. Frontline German Waffen-SS had blood group tattoos during World War II.
Rare blood types can cause supply problems for blood banks and hospitals. For example, Duffy-negative blood occurs much more frequently in people of African origin, and the rarity of this blood type in the rest of the population can result in a shortage of Duffy-negative blood for these patients. Similarly, for RhD negative people there is a risk associated with travelling to parts of the world where supplies of RhD-negative blood are rare, particularly East Asia, where blood services may endeavour to encourage Westerners to donate blood.
Hemolytic disease of the newborn (HDN)
A pregnant woman may carry a fetus with a blood type that is different from her own. Typically, this is an issue if a RhD negative mother has a child with a RhD positive father, and the fetus ends up being Rh positive. In those cases, the mother can make IgG blood group antibodies. This can happen if some of the fetus's blood cells pass into the mother's blood circulation (e.g., a small fetomaternal hemorrhage at the time of childbirth or obstetric intervention), or sometimes after a therapeutic blood transfusion. This can cause hemolytic disease of the newborn (HDN) in the current pregnancy or subsequent pregnancies. Sometimes this is lethal for the fetus; in these cases it is called hydrops fetalis. If a pregnant woman is known to have anti-D antibodies, the RhD blood type of a fetus can be tested by analysis of fetal DNA in maternal plasma to assess the risk to the fetus of Rh disease. Cell-free DNA testing can determine the fetal RHD genotype in a sample of maternal plasma after 10 weeks of gestation. One of the major advances of twentieth-century medicine was to prevent this disease by stopping the formation of anti-D antibodies by D-negative mothers with an injectable medication called Rho(D) immune globulin. Antibodies associated with some blood groups can cause severe HDN, others can only cause mild HDN and others are not known to cause HDN.
Blood products