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.
Blood group systems
A total of 30 human blood group systems are now recognized by the International Society of Blood Transfusion (ISBT).[2] 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,[4] 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.[5][6][7][8] An example of this rare phenomenon is the case of Demi-Lee Brennan, an Australian citizen, whose blood group changed after a liver transplant.[9][10] 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.[11] Certain blood types may affect susceptibility to infections, an example being the resistance to specific malaria species seen in individuals lacking the Duffy antigen.[12] The Duffy antigen, presumably as a result of natural selection, is less common in ethnic groups from areas with a high incidence of malaria.
Main article: ABO blood group system
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.[14]ABO and Rh distribution by country
Country  | Population[17] | O+ | A+ | B+ | AB+ | O- | A- | B- | AB- |
|---|---|---|---|---|---|---|---|---|---|
| Australia[18] | 21,262,641 | 40% | 31% | 8% | 2% | 9% | 7% | 2% | 1% |
| Austria[19] | 8,210,281 | 30% | 33% | 12% | 6% | 7% | 8% | 3% | 1% |
| Belgium[20] | 10,414,336 | 38% | 34% | 8.5% | 4.1% | 7% | 6% | 1.5% | 0.8% |
| Brazil[21] | 198,739,269 | 36% | 34% | 8% | 2.5% | 9% | 8% | 2% | 0.5% |
| Canada[22] | 33,487,208 | 39% | 36% | 7.6% | 2.5% | 7% | 6% | 1.4% | 0.5% |
| Denmark[23] | 5,500,510 | 35% | 37% | 8% | 4% | 6% | 7% | 2% | 1% |
| Estonia[24] | 1,299,371 | 30% | 31% | 20% | 6% | 4.5% | 4.5% | 3% | 1% |
| Finland[25] | 5,250,275 | 27% | 38% | 15% | 7% | 4% | 6% | 2% | 1% |
| France[26] | 62,150,775 | 36% | 37% | 9% | 3% | 6% | 7% | 1% | 1% |
| Germany[27] | 82,329,758 | 35% | 37% | 9% | 4% | 6% | 6% | 2% | 1% |
| Hong Kong SAR[28] | 7,055,071 | 40% | 26% | 27% | 7% | 0.31% | 0.19% | 0.14% | 0.05% |
| Iceland[29] | 306,694 | 47.6% | 26.4% | 9.3% | 1.6% | 8.4% | 4.6% | 1.7% | 0.4% |
| India[30] | 1,166,079,217 | 36.5% | 22.1% | 30.9% | 6.4% | 2.0% | 0.8% | 1.1% | 0.2% |
| Ireland[31] | 4,203,200 | 47% | 26% | 9% | 2% | 8% | 5% | 2% | 1% |
| Israel[32] | 7,233,701 | 32% | 34% | 17% | 7% | 3% | 4% | 2% | 1% |
| Netherlands[33] | 16,715,999 | 39.5% | 35% | 6.7% | 2.5% | 7.5% | 7% | 1.3% | 0.5% |
| New Zealand[34] | 4,213,418 | 38% | 32% | 9% | 3% | 9% | 6% | 2% | 1% |
| Norway[35] | 4,660,539 | 34% | 42.5% | 6.8% | 3.4% | 6% | 7.5% | 1.2% | 0.6% |
| Poland[36] | 38,482,919 | 31% | 32% | 15% | 7% | 6% | 6% | 2% | 1% |
| Portugal[37] | 10,707,924 | 36.2% | 39.8% | 6.6% | 2.9% | 6.0% | 6.6% | 1.1% | 0.5% |
| Saudi Arabia[38] | 28,686,633 | 48% | 24% | 17% | 4% | 4% | 2% | 1% | 0.23% |
| South Africa[39] | 49,320,000 | 39% | 32% | 12% | 3% | 7% | 5% | 2% | 1% |
| Spain[40] | 40,525,002 | 36% | 34% | 8% | 2.5% | 9% | 8% | 2% | 0.5% |
| Sweden[41] | 9,059,651 | 32% | 37% | 10% | 5% | 6% | 7% | 2% | 1% |
| Turkey[42] | 76,805,524 | 29.8% | 37.8% | 14.2% | 7.2% | 3.9% | 4.7% | 1.6% | 0.8% |
| United Kingdom[43] | 61,113,205 | 37% | 35% | 8% | 3% | 7% | 7% | 2% | 1% |
| United States[44] | 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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