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ABO Discrepancies Related to Subgroups of A

ABO subgroups represent phenotypes that demonstrate weaker or variable reactions when tested with polyclonal antisera, such as anti-A and anti-B. For example, the antisera generally used for A forward typing is polyclonal, containing both anti-A and anti-A1. Therefore, it can detect various subgroups of A, with the most commonly encounter group being A1.
Subgroups of A arise due to the incomplete conversion of the H antigen by the associated glycosyltransferase ∝-3-N-acetylgalactosaminyltransferase. Since the A and H antigens typically have an inverse relationship, the less A antigen sites converted equates to more unconverted H precursor on the red blood cell membrane.
Generally, A subgroups do not pose a problem on the blood bank bench unless the patient demonstrates naturally occurring anti-A1 or has been stimulated (by transfusion or pregnancy) to produce anti-A1. Patients demonstrating this reactivity pattern will have ABO testing that resembles the following:

Anti-A Anti-BA1 cells
B cells
Antibody Screen
Patient Results
In this case, we have a discrepancy between the forward and reverse types - the forward type potentially having an A subgroup while the reverse type demonstrates an antibody to A1 reagent red blood cells.