Immunology

Description

Acquired B state should be considered when the serum of a patient contains anti-B and the patient's RBC appears to be group AB with a weak B antigen. The acquired B phenotype arises through the modification of the A antigen by microbial enzymes called deacetylases.

The enzymes modify cellular A immunodominant sugars (GalNAc) so they become more like the B sugar (Gal). A RBCs are the only group that exhibits acquired B activity in vivo. When present in sufficient numbers, acquired B antigens react with human anti-B in direct agglutination tests. While many examples of RBC with acquired B antigens react weakly with anti-B, some examples can be found that are agglutinated quite strongly.

Discussion

To confirm that group A1 RBCs carry the acquired B structure:

• Check the patient's diagnosis. Acquired B antigens tend to be associated with carcinoma of the colon or rectum, infection with gram-negative organisms and intestinal obstructions.
• Test the patient's serum against his or her own RBCs. The anti-B in the patient's serum will not agglutinate his or her own rbcs when they carry the acquired B determinant.
• Test the RBCs with monoclonal anti-B. Some monoclonal reagents, unlike human-source antibodies, do not react with the acquired B phenotype - Such information may be carried in the instructions that accompany the monoclonal reagent. Test the RBCs with human anti-B serum. acidified to pH 6.0. Acidified anti-B sera do not react with the acquired B receptor.
• If the patient is a secretor, test saliva for the presence of A and B substances. Patients whose RBCs carry acquired B structures will have A, but not B, substance in their saliva.

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