ABO blood group system - The Acquired B phenotype

Routine ABO testing is performed in two distinct (but usually simultaneous) stages, known as "red cell grouping" (forward grouping or "front type") and "serum grouping" (reverse grouping or "back type"). Here's an example of how it works: If a person's red blood cells (RBCs) react strongly with reagent anti-A but not anti-B, we would interpret their red cell grouping as blood group A. If there is no ABO discrepancy, that same person's serum should have no reaction with reagent group A1 RBCs and strong reaction with reagent group B RBCs (demonstrating the expected presence of anti-B in the serum). Thus, the serum grouping interpretation would also be blood group A, and no ABO discrepancy would exist

ABO discrepancies occur any time the interpretations of a person's red cell and serum grouping do not agree. ABO discrepancy takes on many forms, and acquired B is a great, if not terribly common, example.

Read more:
Transfusion Medicine Education

Source: Blood Bank Guy