A₁ and A₂ Subgroups

These are the most important subgroups in the system.

Frequency: A₁ = 80% and A₂ = 20%

This means that all other subgroups must be rare.

Differences between A₁ and A₂: These subgroups have a quantitative difference and perhaps a qualitative difference. The quantitative difference: A₁ red cells have about one million A antigens per cell and A₂ red cells have about one-quarter million A antigens per cell. This is because the N-acetyl- D-galactosaminyl transferase made by the A¹ gene is more efficient at converting H antigen into A antigen than is the transferase made by the A² gene. The qualitative difference is controversial: some workers think that the A₁ and A₂ antigens may be slightly chemically different as shown by the fact that some A₂ people can produce anti-A₁. Others believe the difference to be strictly quantitative, i.e., the anti-A₁ made by A₂ persons is really an autoantibody that will not react visibly with the patient's own cells because of too few A antigenic sites.

Development at birth: The A₁ antigen is very poorly developed at birth and newborns usually type as A₂.

Significance: Usually we do not care whether a group A person is A₁ or A₂. In routine tests both react well with anti-A. Sometimes, however, a group A₂ or A₂B person will produce an anti-A₁ which results in an ABO discrepancy that must be resolved (Table 4-3).

Table 4-3. ABO discrepancies due to anti-A₁.

About 1-8% of group A₂ people and 22-35% of A₂B people will make anti-A₁. When this happens we must resolve the discrepancy in order to determine the person's true ABO group. (For simplicity, consider about 2% of A₂ and 25% of A₂B people to make anti-A₁.)

NOTE: Usually anti-A₁ produced by A₂ people is a harmless IgM cold agglutinin that reacts best at 4°C, may react at RT (in the ABO group), but will not react at 37°C. Rarely, anti-A₁ can be clinically significant and react at 37°C.

Resolution of discrepancies caused by anti-A₁

A1 and A2 Subgroups