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

Background: 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. Materials and Methods: 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. Results: 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. Conclusion: 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.