The silent threat: a true Bombay phenotype with unexpected tolerance to group O-positive incidental transfusion

Bombay (Oh) blood group individuals are typically transfused with Bombay group red blood cells because transfusion with other blood groups can lead to life-threatening hemolysis. The mutations in the FUT1 gene prevent the expression of the H antigen on red blood cells.

A 35-year-old man presented with severe anemia (hemoglobin, 4.8 g/dL) and was transfused with 3 units of O RhD-positive packed red blood cells at an outside hospital without adverse reaction. One month later, his blood group at our center was revealed as O RhD positive with panagglutination in reverse grouping. Anti-H lectin was negative, and crossmatch with O RhD-positive units showed strong incompatibility, indicating Bombay phenotype.

Molecular testing revealed FUT1 c.725T>G homozygosity and FUT2 deletion. Concurrent evaluation revealed plasma cell leukemia with extensive bone marrow infiltration, which may explain the transient immunosuppression and no hemolysis. Bombay-compatible units were procured through the Rare Donor Registry and transfused safely, allowing initiation of chemotherapy.

This case highlights that the absence of a transfusion reaction cannot exclude a true Bombay phenotype, underscoring the importance of molecular confirmation. It also emphasizes the timely coordination with donors and rare donor registries, ensuring the availability of compatible blood for patients with rare blood types.