Abstract: Objective Investigation of clinical, serological, and genotypic characteristics and multidisciplinary tiered management strategies in a case of A₁ antigen loss resulting from passenger lymphocyte syndrome (PLS) following ABO minor-incompatible (B to A₁B) allogeneic hematopoietic stem cell transplantation (allo-HSCT), combined with literature review to provide a reference for early clinical identification and treatment. Methods The patient's ABO blood group was determined using both microcolumn gel and tube methods, with genotyping confirmed by Sanger sequencing. Hemolysis was assessed by monitoring pre- and post-transplant changes in total bilirubin, lactate dehydrogenase (LDH), hemoglobin levels, and reticulocyte percentage. Results Following transplantation, the patient exhibited progressive hemoglobin decline, elevated reticulocyte percentage, hyperbilirubinemia, and increased LDH levels—findings indicating hemolysis. Discrepancies were noted between ABO forward and reverse typing. Genetic sequencing confirmed an ABO*A1.02/ABO*B.01 genotype; however, serological testing revealed a transient loss of A₁ antigen expression. Conclusion This case report of transient A₁ antigen loss with passenger lymphocyte syndrome (PLS) following allo-HSCT demonstrates that PLS diagnosis requires the combination of ABO forward/reverse grouping discrepancy, hemolytic laboratory evidence, and recognition of PLS clinical heterogeneity, further highlighting the value of ABO blood group monitoring in multidisciplinary collaboration.

Key words: Allogeneic hematopoietic stem cell transplantation, ABO incompatibility, Passenger lymphocyte syndrome, Antigen loss