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Acquired B Phenomenon

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 below:


Anti-A Anti-BA1 cells
B cells
Patient Results
4+ 2+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. 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.


Patient Red Cell
Patient Plasma
Resulting Reaction
Patient State/Reaction
A antigen (no B antigen)
Anti-BNegative