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.
The ‘A’ antigen on A1 and A2 subgroup blood cells is named ‘Type 2 A’ antigen; however, A1
subgroup blood cells also have two additional forms of antigen as well, ‘Type 3 A’ and ‘Type 4
A’, neither of which appear on A2 subgroup blood cells.