A blood type is a description an individual's characteristics of red blood cells due to substances (carbohydrates and proteins) on the cell membrane. The two most important classifications to describe blood types in humans are ABO and Rh factor. There are 46 other known antigens, most of which 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.
- Individuals with type A blood have red blood cells with antigen A on their surface and produce antibodies against antigen B in their blood serum. Using the blood compatibility chart below, for example, an A-negative person cannot receive blood except from another A-negative person or from an O-negative person.
- Individuals with type B blood have the opposite arrangement, antigen B on the cell and produce antibodies to substance A in their serum.
- Type AB people have red blood cells with both antigens A and B, and do not produce antibodies against either substance in their serum. Therefore, a person with type AB blood can safely receive any ABO type blood and is called a "universal receiver", but cannot donate blood except to the corresponding AB type people shown in the blood compatibility table below.
- Type O people have red blood cells with neither antigen, but produce antibodies against both types of antigens. Because of this arrangement, type O can be safely given to any person with any ABO blood type. Hence, a person with type O blood is said to be a "universal donor" but cannot receive blood except from the corresponding O type people shown in the blood compatibility table below. Thus, for example, an O-negative person cannot receive blood except from another O-negative person.
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  .
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.
Apart from on red blood cells, the ABO antigen is also expressed on the glycoprotein von Willebrand factor (vWF), which participates in hemostasis (control of bleeding). In fact, blood type O predisposes very slightly to bleeding, as vWF is degraded more rapidly.
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 (an Rh positive donor to an Rh negative recipient) will cause hemolysis. The converse is not true: Rh+ patients do not react to Rh- blood.
Rh disease occurs when an Rh negative mother who has already had an Rh positive child (or an accidental Rh+ blood transfusion) carries another Rh positive child. After the first pregnancy, the mother develops antibodies against Rh+ red blood cells, which cross the placenta and hemolyses the blood of the second child. This reaction doesn't always occur and is less likely to occur if the child carries either the A or B antigen and the mother does not. In the past, Rh incompatibility could result in stillbirth or death of the mother. Rh incompatibility was until recently the most common cause of long term disability in the United States. At first, this was treated by transfusing the blood of infants who survived. At present, it can be treated with certain anti-Rh(+) antisera, the most common of which is Rhogam (anti-D). It can be anticipated by determining the blood type of every child of a RhD- mother; if it is Rh+, the mother is treated with anti-D to prevent development of antibodies against Rh+ red blood cells.
Blood groups are inherited from both parents. The ABO blood type is controlled by a single gene with three alleles: i, A, and B. The gene encodes a glycosyltransferase , an enzyme that modifies the carbohydrate content of the red blood cell antigens. The gene is located on the long arm of the ninth chromosome (9q34).
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 (it is, however, no direct proof of illegitimacy).
Evolutionary biologists theorize that the A allele evolved earliest, followed by O and then B. This chronology accounts for the percentage of people worldwide with each blood type. It is consistent with the accepted patterns of early population movements and varying prevalent blood types in different parts of the world. (For instance, B is very common in populations of Asian descent, but rare in ones of European descent.)
Rh (or the D antigen) is inherited on one locus (on the short arm of the first chromosome, 1p36.2-p34) with two alleles, of which Rh+ is dominant and Rh- recessive. The gene codes for a polypeptide on the red cell membrane. Rh- individuals (dd genotype) do not produce this antigen, and may be sensitized to Rh+ blood.
Two very similar epitopes, Cc and Ee, appear to be closely related to Rh.
The rare individuals with Bombay phenotype do not express substance H 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.
Patients who test as type O may have the Bombay phenotype: they have inherited two recessive alleles of the H gene, (their blood group is Oh and their genotype is "hh"), and so do not produce the "H" protein that is the precursor to the "A" and "B" antigens. It then no longer matters whether the A or B enzymes are present or not, as no A or B antigen can be produced since the precursor antigen is not present.
Despite the designation O, Oh -ve is not a sub-group of any other group, not even O -ve or O +ve. When this Blood group was first encountered, it was found not to be of either group A or B and so was thought to be of Group O. But on further test, it did not match even for O-ve or O+ve because of the absence of Antigen 'h'. The name "Bombay group" originates from the city of Bombay, now known as Mumbai, in India. The blood phenotype was first discovered in Bombay.
Blood donors and blood recipients must have compatible blood types. O- is the universally compatible blood type. The chart below illustrates how people with different blood types can receive or donate other blood (X means compatible). An A- person, for example, can receive either O- or A-, and can donate to people with AB+, AB-, A+ or A- blood.
In general, people with type O Rh- are referred to as universal donors, as their blood can be transfused to anyone in need. It is thus the most highly sought after blood type in blood banks and hospitals. A type AB Rh+ is referred to as a universal receiver because he or she can receive blood of any type.
Blood types are not evenly distributed throughout the human population. O+ is the most common, AB- is the rarest. There are also variations in blood-type distribution within human subpopulations.
Other blood types
Other blood type systems exist to describe the presence or absence of other antigens. Many are named after the patients in whom they were initially encountered.
- Diego positive blood is found only among East Asians and Native Americans.
- MNS systems gives blood types of M, N, and MN. It has use in tests of maternity or paternity.
- Duffy negative blood gives partial immunity to malaria.
- The Lutheran system describes a set of 21 antigens.
- Other systems include Colton, Kell, Kidd, Lewis, Landsteiner-Wiener, P, Yt or Cartwright, XG, Scianna, Dombrock, Chido/Rodgers, Kx, Gerbich, Cromer, Knops, Indian, Ok, Raph, and JMH.
In Japan, a popular belief is that personality is related to blood type. From the preponderance of some blood type in a population, "experts" claim to be able to deduce the character of that population. The "experts" also believe that they can calculate how well the blood types of different people match. A Japanese employer could therefore aim to get a proper mix of blood types among their personnel.
- Landsteiner K. Zur Kenntnis der antifermentativen, lytischen und agglutinierenden Wirkungen des Blutserums und der Lymphe. Zentralblatt Bakteriologie 1900;27:357-62.