Aberrant ABO B Phenotype with Irregular Anti-B Caused by a Para-Bombay FUT1 Mutation

Routine ABO blood group typing for pre-transfusion testing of a male Austrian patient of Far Eastern origin showed discrepant results with an apparently weak blood group B phenotype and irregular anti-B.

ABH phenotyping and cross-matching was done by standard serologic techniques and levels of H expression were determined by flow cytometry. ABO gene sequencing including regulatory regions as well as analysis of FUT1 (H), FUT2 (Secretor), and FUT3 (Lewis) were carried out.

While monoclonal ABO antigen typing indicated blood group O, weak agglutination reactions using polyclonal human anti-B and anti-AB were seen. In reverse typing at room temperature, the plasma was reactive with A1 and A2 RBCs and negative with B and O cells, whereas at 4°C, anti-B reactivity was found. The indirect anti-globulin cross-match of the patient's plasma was positive with group B RBCs and negative with group O RBCs. Sequencing analysis showed the presence of ABO*B.01 (B114) allele and homozygosity for the FUT1 mutation c.551_552delAG. Flow cytometry demonstrated trace amounts of H antigen on the patient's RBCs.

While a functional B allele was found, analysis of FUT1 and FUT2 genes revealed the presence of a rare para-Bombay genotype O_h^B. Interestingly, no anti-H but irregular anti-B was found in the patient's plasma, responsible for the positive cross-match with group B RBCs. Even though very rare and not reported for the European population, the presence of an H-deficient phenotype should be considered when investigating individuals with an unusual ABO blood group type.