The most common subgroups of group A phenotype are A1 and A2. These account for over 99% of individuals classified as Group A. Of this 99%, A1 comprises approximately 80%. Commercial anti-A typing serum does not differentiate between A1 and A2 cells. A1 cells contain "A" antigen and "A1" antigen. A2 is not a unique antigen. It is thought to be an "A" antigen with no "A1" antigen. Several preparations are available that will react with A1 cells but not other subgroups of A. The most commonly used reagent is Anti-A1 lectin, an extract of the plant's seeds, Dolichos biflorus, which has specific anti-A1 activity.
Approximately 4% of individuals who are subgroup A2 have naturally occurring anti-A1 in their serum.
