Conditions associated with the production of bacterial enzymes can result in modification of the immunodominant A group sugar (N-acetyl-D-galactosamine) into a structure that highly resembles the B group sugar structure (D-galactose). When this happens, monoclonal anti-B reagents will strongly agglutinate this structure, resulting in false-positive B reactions. This will disappear once the clinical condition is resolved. An example of the acquired B phenomenon is shown in Table 8.
Table 8. Results from Patient with Acquired B Condition.
| Anti-A | Anti-B | A1 cells | B cells |
| Patient Results | 4+ | 1+ | 0 | 4+ |
This ABO discrepancy can be resolved by testing patient plasma with patient red blood cells. Patients who are truly group A, without any true B antigen, will demonstrate a negative auto control (Table 9). Since there is no B antigen present on the patient's red cell and there is anti-B in patient plasma, no agglutination should form.
Table 9. Resolved Results from Patient with Acquired B Condition. | Patient Red Cell | Patient Plasma | Resulting Reaction |
| Patient State/Reaction | A antigen (no B antigen) | Anti-B | Negative |
An alternative resolution method would be to lower the pH of the anti-B reaction; acquired B does not react at a low pH.
