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 A_x 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 red cells and anti-A in his serum. Anti-A₁ is produced by about 1-2% of group A₂ persons and about 25% of group A₂B persons. Subgroups may be determined by reactions with antisera as seen in the table on the next page.