It may be a surprise to find that A, B, O and AB have various subgroups. The A blood type appears to have the most variation in subgroups; several have been found for O's; B subgroups exist but are extremely rare; and AB's have a wide variety, as they can inherit all the possibilities of the A group.
Subgroups of the A Blood Type
A blood types have the most variation in subgroup of any of the ABO blood types. There are about 20 different known subgroups. A1 and A2 individuals make up the vast majority of people with A blood type, all other subgroups equal less than 1% of A's.
A1 and A2 Subgroups
These are the most important subgroups in the system. A1 equals approximately 80% of the entire A blood type population, and A2 makes up the remaining 20%, under current data. This means that all other subgroups of A blood type must be rare.
Differences between A1 and A2
A1 red blood cells ( or 'erythrocytes') have about one million A antigens per cell. A2 red cells have only 250,000 A antigens per cell, or one-fourth the amount that A1 cells have. A2 red cells have much more H antigen than do A1 red cells. The amount of H antigen on red cells of the common ABO groups from most to least is O > A2 > B > A2B > A1 > A1B.
Other A subgroups
A3 is a fairly rare subgroup (1/1000). The main distinguishing feature of A3 red cells is MFA (mixed field agglutination) with anti-A and anti-A,B.
Ax (Ao) This is a rare subgroup (1/40,000). The main distinguishing features of Ax cells are: the A antigen is so weak it may only be detectable by using anti-A,B; and anti-A1 is usually present in the serum. If anti-A,B is not used, Ao cells may be mistyped as group O.
Am is a very rare subgroup. The main distinguishing features of Am cells are: no reaction with anti-A or anti-A,B in routine testing, and anti-A1 is not present in the serum.
Some of the other A subgroups are named A4, Abantu, Afinn, Aint (A1-A2 intermediate), Ael, Acl (various genotypes - AO1, AO1var, AO2), Aend, Ay, and Aweak. A subgroups found in African populations are most likely A2, or the rarer Aint, or Abantu.
An evolutionary relationship exists between A1 and A3 and Ael. A3 and Ael are mutations from the A1 cell. A similar evolutionary relationship exists between A2 and Aend and Aweak.
Significance in transfusion medicine
A1 and A2 are interchangeable as far as transfusion purposes are concerned. Both types will react well with anti-A, so a mistake in typing will usually not be made. There is a very small percentage of A2 (from 1-8%) and a larger group of A2B (22-35%) people who produce anti-A1, which gives an incorrect blood typing result for ABO type. Discrepancies of this sort must be resolved when a transfusion is required, it becomes critical at this point as failure to detect a weak subgroup of A may result in a patient or donor being mistyped as group AB or B. They may also be mistyped s group O, but this is not as serious a problem.
Organs of different blood types are not used for transplant purposes due to immunological complications and decreased graft survival. Organs of group O can be used for other types as O is the universal donor, with no antibodies to other ABO blood types. Organs of A, B or AB individuals have antibodies against other types which presents a problem with graft survival. The A2 blood group subtype appears to be an exception to this rule however. A2 livers have been successfully transplanted into O type individuals.