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:

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.