Bombay (Oh) and Para-Bombay are rare variants of the ABO blood group system that carry significant clinical importance. They are characterized by the absence or a marked reduction in the expression of the H antigen on red blood cells (RBCs). This deficiency leads to a failure in the synthesis of A and B antigens, predisposing patients—particularly those with the Bombay phenotype—to developing potent anti-H antibodies, which can cause severe hemolytic transfusion reactions. Objective: The primary goal is to provide clinicians and laboratory specialists with a practical and comprehensive framework to prevent avoidable blood mismatch and improve clinical outcomes for patients suffering from H-deficient phenotypes. Methods: This review summarizes the underlying biology behind FUT1/FUT2 deficiency and highlights its effects on population groups and founder effects. It integrates serological and molecular tools into a practical diagnostic workflow. Furthermore, it outlines management strategies for pre-surgery, obstetrics, and emergencies, and reviews emerging translational methods, such as the enzymatic removal of H from group O red blood cells. Findings: The review emphasizes the critical nature of early engagement with rare donor networks. It identifies the biological mechanisms of H deficiency and provides a structured approach to managing the risks associated with these rare phenotypes, ensuring that high-potency antibodies do not lead to life-threatening transfusion complications. Conclusion: Implementing a practical and comprehensive framework in laboratories is essential to prevent transfusion incompatibility. By utilizing advanced diagnostic tools and early coordination with donor networks, the safety and outcomes of patients with H antigen deficiency can be significantly enhanced.
