A blood type (or blood group) is a characteristic of an individual's red blood cells, defined in terms of specific substances (carbohydrates and proteins) on the cell membrane. All species have their own blood types; however, for the most part, these have not been studied to any significant degree.
The two most important classifications to describe blood types in humans are ABO and the Rhesus factor (Rh factor). There are 46 other known antigens in humans, most of which are much rarer than ABO and Rh. Blood transfusions from incompatible groups can cause an immunological transfusion reaction, resulting in hemolytic anemia, renal failure, shock, and death. The ABO blood types also exist among chimpanzees and bonobos.
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 Sweden and 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] According to [Benes93] it can be partly attributed to the relation among blood types and particular illnesses: apparently, certain blood types give greater (or lesser) resistance to various diseases. For instance, type-O people have lessened resistance to the Black Plague, and therefore type O is less common in European populations.
Another characteristic of blood is Rhesus factor or Rh factor. It is named after the Rhesus monkey, in which the factor was first identified by Karl Landsteiner and Alexander S. Wiener. Individuals either have, or do not have, the Rh factor on the surface of their red blood cells. This is indicated as + or -, and the two groups are described as Rh positive (Rh+) or Rh negative (Rh-), respectively. 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% of the people are type B.
Matching the Rhesus factor is very important, as mismatching (an Rh positive donor to an Rh negative recipient) may cause the production in the recipient of an antibody to the Rh(D) antigen, which could lead to subsequent hemolysis. This is of particular importance in females of or below childbearing age, where any subsequent pregnancy may be affected by the antibody produced. For one-off transfusions, particularly in older males, the use of Rh(D) positive blood in an Rh(D) negative individual (who has no atypical red cell antibodies) may be indicated if it is necessary to conserve Rh(D) negative stocks for more appropriate use. The converse is not true: Rh+ patients do not react to Rh- blood.