B antigen acquired by normal A1 red cells exposed to a patient's serum.

The authors report a patient with the acquired B antigen whose serum contained a factor capable of inducing the acquired B phenotype on normal A1 red cells in vitro. The patient's red cells were not, however, agglutinated by a monoclonal anti-B. These findings and the abolition of the acquired B antigen by treatment of the patient's red cells with acetic anhydride support the deacetylation theory of the genesis of the acquired B antigen.