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Blood type

A blood type is a description of certain characteristics of blood which depend on certain substances present on the surface of red blood cells. There are 46 known antigens, each of which is described by its own system.

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.

Table of contents
2 Rhesus
3 Inheritance
4 Other blood types


Individuals with type A blood, have red blood cells with substance A on their surface and antibodies against substance B in their blood serum.

Individuals with type B blood have the opposite arrangement, substance B in the cell and antibodies to substance A in their serum. Type O people have neither substance but can form antibodies against both types. Type AB people have both substances. 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". A type AB person can safely receive any ABO type blood and is called a "universal receiver".

The precise reason why people are born with antibodies against an antigen they have never been exposed to is unknown. It is thought that some bacterial antigens are similar enough to the A and B glycoproteins, 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. 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.

Rh factor is named after the rhesus monkey where the factor was first identified.


Blood groups are inherited from both parents. The ABO blood type is controlled by a single gene with three alleles: 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.

Bombay phenotype

Another possible explanation is that the child or parent who tests as type O has the very rare 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.

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, a 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.

Rhesus inheritance

Rh is inherited the same way, except that it has two alleles and Rh is dominant. Rh Disease is caused by an Rh negative mother having an Rh positive child. The antibodies in the mother's blood destroy the infant's blood. At first, this was treated by transfusing the blood of infants who survived.

Other blood types

Other blood type systems exist to describe the presence or absence of other antigens. 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, Hh or Bombay, Kell, Kidd, Lewis, Landsteiner-Wiener, P, Yt or Cartwright, XG, Scianna, Dombrock, Chido/Rodgers, Kx, Gerbich, Cromer, Knops, Indian, Ok, Raph, and JMH.