A blood type is a description of 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 the Rhesus factor (Rh factor). There are 46 other known antigens, 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 phrases "blood group" and "blood type" are often used interchangeably, although this is not technically correct. "Blood group" is used to refer specifically to a person's ABO status, while "blood type" refers to both ABO and Rh factors.

Blood type is determined by the antigens (epitopes) on the surface of a red blood cell. Some of these are proteins, while others are proteins with polysaccharides attached. The absence of some of these markers leads to production of antibodies against this marker. The exact reason why this happens is poorly understood, as generally an antigen needs to be present to elicit an immune response. Administration of the wrong blood type would lead to immediate destruction of the infused blood. The breakdown products cause acute medical illness; hence, it is of vital importance that the correct blood type of the donor and receptor is determined, and their blood properly matched.

Austrian scientist Karl Landsteiner is widely credited with the discovery of the main blood type system (ABO); he was awarded the Nobel Prize in Physiology or Medicine in 1930 for his work.

ABO System

Humans have the following blood types along with their respective antigens and antibodies:

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. Therefore an A-negative person can only receive blood from another A-negative person or from an O-negative person.

Individuals with type B blood have the opposite arrangement, antigen B on their cells and produce antibodies against antigen A in their serum. Therefore, a B-negative person can only receive blood from another B-negative person or from an O-negative person.

Individuals with type AB blood have red blood cells with both antigens A and B and do not produce antibodies against either antigen in their serum. Therefore, a person with type AB-positive blood can safely receive any ABO type blood and is called a "universal receiver". However an AB-positive person cannot donate blood except to another AB-positive person.

Individuals with type O blood have red blood cells with neither antigen but produce antibodies against both types of antigens. Therefore, a person with type O-negative blood can safely donate to a person with any ABO blood type and is called a "universal donor". However an O-negative person can only receive blood 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 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.

The precise reason why people develop 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 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. ABO antigens are also present in many other tissues such as liver, kidneys and lungs.

The H antigen

The A & B antigens are derived from a common precursor known as the H antigen. The H antigen is a glycosphingolipid (sphingolipid with carbohydrates bonded to the ceramide moiety) which is modified to produce the A and B antigens. In type O blood, it remains unchanged and consists of a chain of glucose, galactose, N-acetyl galactosamine, and fucose attached to the ceramide.

Rhesus system (CDE)

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 Landsteiner. Someone either has or does 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.

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.

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 IgG antibodies against Rh+ red blood cells, which can cross the placenta and hemolyze the red cells 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. At present, it can be treated with certain anti-Rh(+) antisera, the most common of which is Rhogam (anti-D).

ABO blood type incompatibilities between the mother and child do not cause a similar problem because antibodies to the ABO blood groups are of the IgM type, which do not cross the placenta.

Frequency of ABO and Rhesus

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. The figures given here are for people of European descent.

Type      Frequency

O+     38%

A+     34%

B+     9%

O-     7%

A-     6%

AB+   3%

B-     2%

AB-   1%

Other Blood Types

There are 27 other blood type systems that exist to describe the presence or absence of other antigens. They exist alongside the ABO antigens, and hence one can be A Rh positive but in addition have Kell or Lewis positivity or negativity.

Diego positive blood is found only among East Asians and Native Americans.

MNS system 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, and is found within African populations.

The Lutheran system describes a set of 21 antigens.

Duffy-type blood presents special problems for blood donation groups and recipients because it occurs in a relatively small segment of the African population, but can cause problems if the recipient isn't properly matched with Duffy-type blood.

Bombay Phenotype

The rare individuals with Bombay phenotype (hh) 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 genotype) 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. Those anticipating the need for blood transfusion (e.g. in scheduled surgery) may bank blood for their own use (i.e. an autologous blood donation) but this option is not available in cases of accidental injury. The blood phenotype was first discovered in Bombay, now known as Mumbai, in India.

Blood Type