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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, and some of these antigens are also present on the surface of other types of cell 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 which ranges from mild to severe.

Serology

If an individual is exposed to a blood group antigen that is not recognized as self, the immune system will produce antibodies that can specifically bind to that particular blood group antigen, and an immunological memory against that antigen is formed. The individual will have become sensitized to that blood group antigen. These antibodies can bind to antigens on the surface of transfused red blood cells (or other tissue cells), often leading to destruction of the cells by recruitment of other components of the immune system. When IgM antibodies bind to the transfused cells, the transfused cells can clump. It is vital that compatible blood is selected for transfusions and that compatible tissue is selected for organ transplantation. Transfusion reactions involving minor antigens or weak antibodies may lead to minor problems. However, more serious incompatibilities can lead to a more vigorous immune response with massive RBC destruction, low blood pressure, and even death.

ABO and Rh blood grouping

Agglutination of blood cells tested with antibodies for determination of blood type in the laboratory.
The discovery of this type of agglutination was an important medical breakthrough.
Anti-A and Anti-B, the common IgM antibodies to the RBC surface antigens of the ABO blood group system, are sometimes described as being "naturally occurring"; however, this is a misnomer, because these antibodies are formed in infancy by sensitization in the same way as other antibodies. The theory that explains how these antibodies are developed states that antigens similar to the A and B antigens occur in nature, including in food, plants, and bacteria. After birth an infant's gut becomes colonized with normal flora that express these A-like and B-like antigens, causing the immune system to make antibodies to those antigens that the red blood cells do not possess. People who are blood type A will have Anti-B antibodies, blood type B will have Anti-A antibodies, blood type O will have both Anti-A and Anti-B antibodies, and blood type AB will have neither. Because of these so called "naturally occurring" and expected antibodies, it is important to correctly determine a patient's blood type prior to transfusion of any blood component. These "naturally occurring" antibodies are of the IgM class, which have the capability of agglutinating (clumping) and damaging red blood cells within the blood vessels, possibly leading to death. It is not necessary to determine any other blood groups because almost all other red blood cell antibodies can develop only through active immunization, which can occur only through either previous blood transfusion or pregnancy. A test called the Antibody Screen is always performed on patients who may require red blood cell transfusion, and this test will detect most clinically significant red blood cell antibodies.

The RhD antigen is also important in determining a person's blood type. The terms "positive" or "negative" refer to either the presence or absence of the RhD antigen irrespective of the presence or absence of the other antigens of the Rhesus system. Anti-RhD is not usually a naturally occurring antibody as the Anti-A and Anti-B antibodies are. Cross-matching for the RhD antigen is extremely important, because the RhD antigen is immunogenic, meaning that a person who is RhD negative is very likely to make Anti-RhD when exposed to the RhD antigen (perhaps through either transfusion or pregnancy). Once an individual is sensitized to RhD antigens, his or her blood will contain RhD IgG antibodies, which can bind to RhD positive RBCs and may cross the placenta.

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 (eg, 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.

ABO blood group system

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/null) in other languages.
Phenotype Genotype
A AA or AO
B BB or BO
AB AB
O OO


Rhesus blood group system

The Rhesus system is the second most significant blood-group system in human-blood transfusion. The most significant Rhesus antigen is the RhD antigen because it is the most immunogenic of the five main rhesus antigens. It is common for RhD-negative individuals not to have any anti-RhD IgG or IgM antibodies, because anti-RhD antibodies are not usually produced by sensitization against environmental substances. However, RhD-negative individuals can produce IgG anti-RhD antibodies following a sensitizing event: possibly a fetomaternal transfusion of blood from a fetus in pregnancy or occasionally a blood transfusion with RhD positive RBC. Rh disease can develop in these cases.

ABO and Rh distribution by country

ABO and Rh blood type distribution by nation (population averages)
Country Population  O+  A+  B+ AB+  O-  A-  B- AB-
Australia 21,262,641 40% 31% 8% 2% 9% 7% 2% 1%
Austria 8,210,281 30% 33% 12% 6% 7% 8% 3% 1%
Belgium 10,414,336 38% 34% 8.5% 4.1% 7% 6% 1.5% 0.8%
Brazil 198,739,269 36% 34% 8% 2.5% 9% 8% 2% 0.5%
Canada 33,487,208 39% 36% 7.6% 2.5% 7% 6% 1.4% 0.5%
Denmark 5,500,510 35% 37% 8% 4% 6% 7% 2% 1%
Estonia 1,299,371 30% 31% 20% 6% 4.5% 4.5% 3% 1%
Finland 5,250,275 27% 38% 15% 7% 4% 6% 2% 1%
France 62,150,775 36% 37% 9% 3% 6% 7% 1% 1%
Germany 82,329,758 35% 37% 9% 4% 6% 6% 2% 1%
Hong Kong SAR 7,055,071 40% 26% 27% 7% 0.31% 0.19% 0.14% 0.05%
Iceland 306,694 47.6% 26.4% 9.3% 1.6% 8.4% 4.6% 1.7% 0.4%
India 1,166,079,217 36.5% 22.1% 30.9% 6.4% 2.0% 0.8% 1.1% 0.2%
Ireland 4,203,200 47% 26% 9% 2% 8% 5% 2% 1%
Israel 7,233,701 32% 34% 17% 7% 3% 4% 2% 1%
New Zealand 4,213,418 38% 32% 9% 3% 9% 6% 2% 1%
Norway 4,660,539 34% 42.5% 6.8% 3.4% 6% 7.5% 1.2% 0.6%
Poland 38,482,919 31% 32% 15% 7% 6% 6% 2% 1%
Portugal 10,707,924 36.2% 39.8% 6.6% 2.9% 6.0% 6.6% 1.1% 0.5%
Saudi Arabia 28,686,633 48% 24% 17% 4% 4% 2% 1% 0.23%
South Africa 49,320,000 39% 32% 12% 3% 7% 5% 2% 1%
Spain 40,525,002 36% 34% 8% 2.5% 9% 8% 2% 0.5%
Sweden 9,059,651 32% 37% 10% 5% 6% 7% 2% 1%
Netherlands 16,715,999 39.5% 35% 6.7% 2.5% 7.5% 7% 1.3% 0.5%
Turkey 76,805,524 29.8% 37.8% 14.2% 7.2% 3.9% 4.7% 1.6% 0.8%
United Kingdom 61,113,205 37% 35% 8% 3% 7% 7% 2% 1%
United States 307,212,123 37.4% 35.7% 8.5% 3.4% 6.6% 6.3% 1.5% 0.6%
.
Population-weighted mean (total population = 2,261,025,244) 36.44% 28.27% 20.59% 5.06% 4.33% 3.52% 1.39% 0.45%



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