D-- phenotype and anti-Rh17 alloimmunization: a comprehensive review of identification and clinical management

The D-- phenotype is an exceptionally rare Rh blood group phenotype characterized by expression of D in the complete absence of C, c, E, and e. Individuals with this phenotype are at risk of developing anti-Rh17 (anti-Hr₀), a clinically significant alloantibody that can severely complicate transfusion support and pregnancy management because compatible blood is exceptionally difficult to obtain. This review critically synthesizes the available published evidence regarding the D-- phenotype and anti-Rh17 alloimmunization, integrating data from case reports, case series, family investigations, and molecular studies from different geographic regions. Current evidence regarding molecular mechanisms, immunohematologic character istics, serologic identification, transf usion management, pregnancy outcomes, and rare donor strategies is summarized, w ith emphasis on practical considerations relevant to transfusion medicine. Although the D-- phenotype is exceptionally rare, early recognition, comprehensive immunohematologic evaluation, and proactive transfusion planning, including collaboration with rare donor programs, are essential for optimizing patient outcomes.