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Blood type (or blood group) is determined, in part, by the ABO blood group antigens present on red blood cells.
A blood type (also called a blood group) is a classification of blood based on the presence or absence of 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 system. 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 that stem from one allele (or very closely linked genes), collectively form a blood group system. Blood types are inherited and represent contributions from both parents. A total of 30 human blood group systems are now recognized by the International Society of Blood Transfusion (ISBT).

Many pregnant women carry a fetus with a different blood type from their own, and the mother can form antibodies against fetal RBCs. Sometimes these maternal antibodies are IgG, a small immunoglobulin, which can cross the placenta and cause hemolysis of fetal RBCs, which in turn can lead to hemolytic disease of the newborn, an illness of low fetal blood counts that ranges from mild to severe.
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Blood group systems
A total of 30 human blood group systems are now recognized by the International Society of Blood Transfusion (ISBT). A complete blood type would describe a full set of 30 substances on the surface of RBCs, and an individual's blood type is one of the many possible combinations of blood-group antigens. Across the 30 blood groups, over 600 different blood-group antigens have been found, but many of these are very rare or are mainly found in certain ethnic groups.

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. An example of this rare phenomenon is the case of Demi-Lee Brennan, an Australian citizen, whose blood group changed after a liver transplant. Another more common cause in 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 who is a different ABO type (e.g., a type A patient receives a type O bone marrow), the patient's blood type will eventually convert to the donor's type.

Some blood types are associated with inheritance of other diseases; for example, the Kell antigen is sometimes associated with McLeod syndrome. Certain blood types may affect susceptibility to infections, an example being 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 ethnic groups from areas with a high incidence of malaria.
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ABO blood group system
NARPO ABO blood group system: diagram showing the carbohydrate chains that determine the ABO blood group

The ABO system is the most important blood-group system in human-blood transfusion. The associated anti-A antibodies and anti-B antibodies are usually Immunoglobulin M, abbreviated IgM, antibodies. ABO IgM antibodies are produced in the first years of life by sensitization to environmental substances such as food, bacteria, and viruses. The O in ABO is often called 0 (zero, or null) in other languages.

Phenotype Genotype
A AA or AO
B BB or BO
AB AB
O OO
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Rh blood group system
The Rh system 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 disease can develop in these cases. Rh negative blood types are much less in proportion of Asian populations (0.3%) than they are in White (15%).

Racial and ethnic distribution of ABO (without Rh) blood types (This table has more entries than the table above but does not distinguish between Rh types.)

ABO and Rh blood type distribution by nation (population averages)
Country  O+  A+  B+ AB+  O-  A-  B- AB-
Australia 40% 31% 8% 2% 9% 7% 2% 1%
Austria 30% 33% 12% 6% 7% 8% 3% 1%
Belgium 38% 34% 8.5% 4.1% 7% 6% 1.5% 0.8%
Brazil 36% 34% 8% 2.5% 9% 8% 2% 0.5%
Canada 39% 36% 7.6% 2.5% 7% 6% 1.4% 0.5%
Denmark 35% 37% 8% 4% 6% 7% 2% 1%
Estonia 30% 31% 20% 6% 4.5% 4.5% 3% 1%
Finland 27% 38% 15% 7% 4% 6% 2% 1%
France 36% 37% 9% 3% 6% 7% 1% 1%
Germany 35% 37% 9% 4% 6% 6% 2% 1%
Hong Kong SAR 40% 26% 27% 7% 0.31% 0.19% 0.14% 0.05%
Iceland 47.6% 26.4% 9.3% 1.6% 8.4% 4.6% 1.7% 0.4%
India 36.5% 22.1% 30.9% 6.4% 2.0% 0.8% 1.1% 0.2%
Ireland 47% 26% 9% 2% 8% 5% 2% 1%
Israel 32% 34% 17% 7% 3% 4% 2% 1%
Netherlands 39.5% 35% 6.7% 2.5% 7.5% 7% 1.3% 0.5%
New Zealand 38% 32% 9% 3% 9% 6% 2% 1%
Norway 34% 42.5% 6.8% 3.4% 6% 7.5% 1.2% 0.6%
Poland 31% 32% 15% 7% 6% 6% 2% 1%
Portugal 36.2% 39.8% 6.6% 2.9% 6.0% 6.6% 1.1% 0.5%
Saudi Arabia 48% 24% 17% 4% 4% 2% 1% 0.23%
South Africa 39% 32% 12% 3% 7% 5% 2% 1%
Spain 36% 34% 8% 2.5% 9% 8% 2% 0.5%
Sweden 32% 37% 10% 5% 6% 7% 2% 1%
Taiwan 43.9% 25.9% 23.9% 6.0% 0.1% 0.1% 0.01% 0.02%
Turkey 29.8% 37.8% 14.2% 7.2% 3.9% 4.7% 1.6% 0.8%
United Kingdom 37% 35% 8% 3% 7% 7% 2% 1%
United States 37.4% 35.7% 8.5% 3.4% 6.6% 6.3% 1.5% 0.6%
Population-weighted mean 36.44% 28.27% 20.59% 5.06% 4.33% 3.52% 1.39% 0.45%
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ABO and Rh distribution by country
People group O (%) A (%) B (%) AB (%)
Aborigines 61 39 0 0
Abyssinians 43 27 25 5
Ainu (Japan) 17 32 32 18
Albanians 38 43 13 6
Grand Andamanese 9 60 23 9
Arabs 34 31 29 6
Armenians 31 50 13 6
Asian (in USA—general) 40 28 27 5
Austrians 36 44 13 6
Bantus 46 30 19 5
Basques 51 44 4 1
Belgians 47 42 8 3
Blackfoot (N. Am. Indian) 17 82 0 1
Bororo (Brazil) 100 0 0 0
Brazilians 47 41 9 3
Bulgarians 32 44 15 8
Burmese 36 24 33 7
Buryats (Siberia) 33 21 38 8
Bushmen 56 34 9 2
Chinese-Canton 46 23 25 6
Chinese-Peking 29 27 32 13
Chuvash 30 29 33 7
Czechs 30 44 18 9
Danes 41 44 11 4
Dutch 45 43 9 3
Egyptians 33 36 24 8
English 47 42 9 3
Eskimos (Alaska) 38 44 13 5
Eskimos (Greenland) 54 36 23 8
Estonians 34 36 23 8
Fijians 44 34 17 6
Finns 34 41 18 7
French 43 47 7 3
Georgians 46 37 12 4
Germans 41 43 11 5
Greeks 40 42 14 5
Gypsies (Hungary) 29 27 35 10
Hawaiians 37 61 2 1
Hindus (Bombay) 32 29 28 11
Hungarians 36 43 16 5
Icelanders 56 32 10 3
Indians (India—general) 37 22 33 7
Indians (USA—general) 79 16 4 1
Irish 52 35 10 3
Italians (Milan) 46 41 11 3
Japanese 30 38 22 10
Jews (Germany) 42 41 12 5
Jews (Poland) 33 41 18 8
Kalmuks 26 23 41 11
Kikuyu (Kenya) 60 19 20 1
Koreans 28 32 31 10
Lapps 29 63 4 4
Latvians 32 37 24 7
Lithuanians 40 34 20 6
Malaysians 62 18 20 0
Maori 46 54 1 0
Mayas 98 1 1 1
Moros 64 16 20 0
Navajo (N. Am. Indian) 73 27 0 0
Nicobarese (Nicobars)  74 9 15 1
Norwegians 39 50 8 4
Papuas (New Guinea) 41 27 23 9
Persians 38 33 22 7
Peru (Indians) 100 0 0 0
Filipinos 45 22 27 6
Poles 33 39 20 9
Portuguese 35 53 8 4
Romanians 34 41 19 6
Russians 33 36 23 8
Sardinians 50 26 19 5
Scots 51 34 12 3
Serbians 38 42 16 5
Shompen (Nicobars) 100 0 0 0
Slovaks 42 37 16 5
South Africans 45 40 11 4
Spanish 38 47 10 5
Sudanese 62 16 21 0
Swedes 38 47 10 5
Swiss 40 50 7 3
Tartars 28 30 29 13
Thais 37 22 33 8
Turks 43 34 18 6
Ukrainians 37 40 18 6
USA (US blacks) 49 27 20 4
USA (US whites) 45 40 11 4
Vietnamese 42 22 30 5
Mean 43.91 34.80 16.55 5.14
Standard deviation 16.87 13.80 9.97 3.41

Blood group B has its highest frequency in Northern India and neighboring Central Asia, and its incidence diminishes both towards the west and the east, falling to single digit percentages in Spain. It is believed to have been entirely absent from Native American and Australian Aboriginal populations prior to the arrival of Europeans in those areas. Blood group A is associated with high frequencies in Europe, especially in Scandinavia and Central Europe, although its highest frequencies occur in some Australian Aborigine populations and the Blackfoot Indians of Montana.
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Other blood group systems

The International Society of Blood Transfusion currently recognizes 30 blood-group systems (including the ABO and Rh systems).[2] 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), and so on, being positive or negative for each blood group system antigen. Many of the blood group systems were named after the patients in whom the corresponding antibodies were initially encountered.
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Clinical significance
Blood transfusion NARPO
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 to provide a transfusion service for 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. In the USA, blood products are tightly regulated by the U.S. Food and Drug Administration.

Main symptoms of acute hemolytic reaction due to blood type mismatch. 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 is 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), renal 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. Risks can be further reduced by cross-matching blood, but this may be skipped when blood is required for an emergency. 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, blood bank technicians usually check for agglutination with a microscope. If agglutination occurs, that particular donor's blood cannot be transfused to that particular recipient. In a blood bank it is vital that all blood specimens are correctly identified, so labeling has been standardized using a barcode system known as ISBT 128.

The blood group may be included on identification tags or 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 patients of African ethnicity. 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 endeavor to encourage Westerners to donate blood.
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Hemolytic disease of the newborn (HDN)
A pregnant woman can make IgG blood group antibodies if her fetus has a blood group antigen that she does not have. This can happen if some of the fetus' 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 Rh disease or other forms of hemolytic disease of the newborn (HDN) in the current pregnancy and/or subsequent pregnancies. If a pregnant woman is known to have anti-D antibodies, the Rh 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. 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.[58][59] Antibodies associated with some blood groups can cause severe HDN, others can only cause mild HDN and others are not known to cause HDN.
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Blood products
To provide maximum benefit from each blood donation and to extend shelf-life, blood banks fractionate some whole blood into several products. The most common of these products are packed RBCs, plasma, platelets, cryoprecipitate, and fresh frozen plasma (FFP). FFP is quick-frozen to retain the labile clotting factors V and VIII, which are usually administered to patients who have a potentially fatal clotting problem caused by a condition such as advanced liver disease, overdose of anticoagulant, or disseminated intravascular coagulation (DIC).

Units of packed red cells are made by removing as much of the plasma as possible from whole blood units.

Clotting factors synthesized by modern recombinant methods are now in routine clinical use for hemophilia, as the risks of infection transmission that occur with pooled blood products are avoided.
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Red blood cell compatibility

  1. Blood group AB - individuals have both A and B antigens on the surface of their RBCs, and their blood serum does not contain any antibodies against either A or B antigen. Therefore, an individual with type AB blood can receive blood from any group (with AB being preferable), but can donate blood only to another type AB individual.

  2. Blood group A - Individuals have the A antigen on the surface of their RBCs, and blood serum containing IgM antibodies against the B antigen. Therefore, a group A individual can receive blood only from individuals of groups A or O (with A being preferable), and can donate blood to individuals with type A or AB.

  3. Blood group B - individuals have the B antigen on the surface of their RBCs, and blood serum containing IgM antibodies against the A antigen. Therefore, a group B individual can receive blood only from individuals of groups B or O (with B being preferable), and can donate blood to individuals with type B or AB.

  4. Blood group O - (or blood group zero in some countries) individuals do not have either A or B antigens on the surface of their RBCs, but their blood serum contains IgM anti-A antibodies and anti-B antibodies against the A and B blood group antigens. Therefore, a group O individual can receive blood only from a group O individual, but can donate blood to individuals of any ABO blood group (i.e. A, B, O or AB). If anyone needs a blood transfusion in an extremely dire emergency, and if the time taken to process the recipient's blood would cause a detrimental delay, O Negative blood can be issued.
NARPO

RBC Compatibility chart
In addition to donating to the same blood group; type O blood donors can give to A, B and AB; blood donors of types A and B can give to AB.

Red blood cell compatibility table
Recipient[1] Donor[1]
O− O+ A− A+ B− B+ AB− AB+
O− Green tick
O+ Green tick Green tick
A− Green tick Green tick
A+ Green tick Green tick Green tick Green tick
B− Green tick Green tick
B+ Green tick Green tick Green tick Green tick
AB− Green tick Green tick Green tick Green tick
AB+ Green tick Green tick Green tick Green tick Green tick Green tick Green tick Green tick

Table note
Assumes absence of atypical antibodies that would cause an incompatibility between donor and recipient blood, as is usual for blood selected by cross matching. An Rh D-negative patient who does not have any anti-D antibodies (never being previously sensitized to D-positive RBCs) can receive a transfusion of D-positive blood once, but this would cause sensitization to the D antigen, and a female patient would become at risk for hemolytic disease of the newborn. If a D-negative patient has developed anti-D antibodies, a subsequent exposure to D-positive blood would lead to a potentially dangerous transfusion reaction. Rh D-positive blood should never be given to D-negative women of child bearing age or to patients with D antibodies, so blood banks must conserve Rh-negative blood for these patients. In extreme circumstances, such as for a major bleed when stocks of D-negative blood units are very low at the blood bank, D-positive blood might be given to D-negative females above child-bearing age or to Rh-negative males, providing that they did not have anti-D antibodies, to conserve D-negative blood stock in the blood bank. The converse is not true; Rh D-positive patients do not react to D negative blood. This same matching is done for other antigens of the Rh system as C, c, E and e and for other blood group systems with a known risk for immunization such as the Kell system in particular for females of child-bearing age or patients with known need for many transfusions.
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Plasma compatibility
Plasma compatibility chart
NARPO In addition to donating to the same blood group; plasma from type AB can be given to A, B and O; plasma from types A and B can be given to O. Recipients can receive plasma of the same blood group, but otherwise the donor-recipient compatibility for blood plasma is the converse of that of RBCs: plasma extracted from type AB blood can be transfused to individuals of any blood group; individuals of blood group O can receive plasma from any blood group; and type O plasma can be used only by type O recipients.

Red blood cell compatibility table
Recipient Donor[1]
O A B AB
O Green tick Green tick Green tick Green tick
A Green tick Green tick
B Green tick Green tick
AB Green tick

Table note
Assumes absence of strong atypical antibodies in donor plasma Rh D antibodies are uncommon, so generally neither D negative nor D positive blood contain anti-D antibodies. If a potential donor is found to have anti-D antibodies or any strong atypical blood group antibody by antibody screening in the blood bank, they would not be accepted as a donor (or in some blood banks the blood would be drawn but the product would need to be appropriately labeled); therefore, donor blood plasma issued by a blood bank can be selected to be free of D antibodies and free of other atypical antibodies, and such donor plasma issued from a blood bank would be suitable for a recipient who may be D positive or D negative, as long as blood plasma and the recipient are ABO compatible.
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Universal donors and universal recipients
With regard to transfusions of whole blood or packed red blood cells, individuals with type O Rh D negative blood are often called universal donors, and those with type AB Rh D positive blood are called universal recipients; however, these terms are only generally true with respect to possible reactions of the recipient's anti-A and anti-B antibodies to transfused red blood cells, and also possible sensitization to Rh D antigens. One exception is individuals with hh antigen system (also known as the Bombay blood group) who can only receive blood safely from other hh donors, because they form antibodies against the H substance.

Blood donors with particularly strong anti-A, anti-B or any atypical blood group antibody are excluded from blood donation. The possible reactions of anti-A and anti-B antibodies present in the transfused blood to the recipients RBCs need not be considered, because a relatively small volume of plasma containing antibodies is transfused.

By way of example: considering the transfusion of O Rh D negative blood (universal donor blood) into a recipient of blood group A Rh D positive, an immune reaction between the recipient's anti-B antibodies and the transfused RBCs is not anticipated. However, the relatively small amount of plasma in the transfused blood contains anti-A antibodies, which could react with the A antigens on the surface of the recipients RBCs, but a significant reaction is unlikely because of the dilution factors. Rh D sensitization is not anticipated.

Additionally, red blood cell surface antigens other than A, B and Rh D, might cause adverse reactions and sensitization, if they can bind to the corresponding antibodies to generate an immune response. Transfusions are further complicated because platelets and white blood cells (WBCs) have their own systems of surface antigens, and sensitization to platelet or WBC antigens can occur as a result of transfusion.

With regard to transfusions of plasma, this situation is reversed. Type O plasma, containing both anti-A and anti-B antibodies, can only be given to O recipients. The antibodies will attack the antigens on any other blood type. Conversely, AB plasma can be given to patients of any ABO blood group due to not containing any anti-A or anti-B antibodies.
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Blood group genotyping
In addition to the current practice of serologic testing of blood types, the progress in molecular diagnostics allows the increasing use of blood group genotyping. In contrast to serologic tests reporting a direct blood type phenotype, genotyping allows the prediction of a phenotype based on the knowledge of the molecular basis of the currently known antigens. This allows a more detailed determination of the blood type and therefore a better match for transfusion, which can be crucial in particular for patients with needs for many transfusions to prevent allo-immunization.
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Conversion
In April 2007, a method was discovered to convert blood types A, B, and AB to O, using enzymes. This method is still experimental and the resulting blood has yet to undergo human trials. The method specifically removes or converts antigens on the red blood cells, so other antigens and antibodies would remain. This does not help plasma compatibility, but that is a lesser concern since plasma has much more limited clinical utility in transfusion and is much easier to preserve.
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