For the most part, subgroups are merely of academic interest, but occasionally they present clinical problems. The antigen may be so weak that it is not detected and the red cells are mistyped as group O. This is especially dangerous if the cells are those of a donor. Problems may arise because the serum of an A2 or A2B, A3, or Ax individual might contain anti-A1. This antibody may be detected in serum typing and cause confusion. You would not expect to find a person with A antigen on his/her red cells and anti-A in the serum. Anti-A1 is produced by about 4% of group A2 individuals and about 25% of group A2B individuals. Subgroups may be determined by reactions with antisera as seen in the table on the next page.