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Two important classifications to describe blood types in humans are ABO and Rh factor. Blood transfusions from incompatible groups can cause an immunological "transfusion reaction", resulting in hemolysis, anemia, renal failure, shock, and death.
Humans have the following blood types along with their respective antigens and antibodies:
Overall, the O blood type is the most common blood type in the world, although in some areas, such as Norway, the A group dominates. The A antigen is overall more common than the B antigen. Since the AB blood type requires the presence of both A and B antigens, the AB blood type is the rarest of the ABO blood types. There are known racial and geographic distributions of the ABO blood types [1].
The precise reason why people are born with antibodies against an antigen they have never been exposed to is unknown. It is believed that some bacterial antigens are similar enough to the A and B glycoproteins, and that antibodies created against the bacteria will react to ABO-incompatible blood cells.
Austrian scientist Karl Landsteiner was awarded the Nobel Prize in Physiology or Medicine in 1930 for his work in discovering ABO blood types.
Another characteristic of blood is Rhesus factor or Rh factor. It is named after the Rhesus Monkey where the factor was first identified in 1940. Someone either has or does not have the Rh factor on the surface of their red blood cells. This is indicated as + or -. This is often combined with the ABO type. Type O+ blood is most common, though in some areas type A prevails, and there are other areas in which as many as 80 percent of the people are type B.
Matching the Rhesus factor in the ABO system is very important, as mismatching (i.e. an Rh positive donor to an Rh negative recipient) will cause hemolysis.
Other important considerations for Rh blood types include situations where a pregnant woman and her fetus are of opposing Rhesus types (one positive, the other negative).
The rare individuals with Bombay phenotype do not express H substance on their red blood cells and therefore do not bind A or B antigens. Instead, they produce antibodies to H substance (which is present on all red cells except those of hh phenotype) as well as to both A and B antigens and are therefore compatible only with other hh donors.
Individuals with Bombay phenotype blood groups can only be transfused with blood from other Bombay phenotype individuals. Given that this condition is very rare to begin with, any person with this blood group, who needs an urgent blood transfusion, may be simply out of luck, as it would be quite unlikely that any blood bank would have any in stock.
Blood groups are inherited from both parents. The ABO blood type is controlled by a single geneDNA and to a chromosome (right). Introns are regions often found in eukaryote genes which are removed in the splicing process: only the exons encode the protein. This diagram labels a region of only 40 or so bases as a gene. In reality many genes are much with three alleleAn allele is any one of a number of alternative forms of the same gene occupying a given locus (position) on a chromosome. An example is the gene for blossom color in many species of flower a single gene controls the color of the petals, but there may bes: i, A, and B.
A allele gives type A, B gives type B, and i gives type O. A and B are dominant over i, so ii people have type O, AA or Ai have A, BB or Bi have type B. AB people have both phenotypes because A and B express a special dominance relationship: codominance. Thus, it is usually impossible for a type AB parent to have a type O child.
When a type AB parent has a type O child, or when one type A and one type O parent produce a type AB child, it is sometimes mistakenly assumed that the child MUST be illegitimate.
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