<p>Refractory immune-mediated cytopenias (IMCs) after allogeneic hematopoietic stem cell transplantation (allo-HSCT) are a challenging complication with limited therapeutic options. Although preexisting human leukocyte antigen (HLA) antibodies, particularly donor-specific antibodies (DSAs), are well-established risk factors for graft failure, the clinical significance and mechanistic role of de novo HLA antibodies emerging after transplantation remain poorly defined. Whether such antibodies are associated with posttransplant IMCs has not been systematically explored. We report two patients with severe aplastic anemia who developed refractory IMCs after allo-HSCT in association with high-titer de novo HLA antibodies and preserved full donor chimerism. In case 1, combined HLA class I and II antibodies were detected in association with trilineage cytopenia. Rituximab, corticosteroids and plasma exchange reduced HLA class II antibodies (including DSAs), whereas persistent HLA class I antibodies were linked to ongoing pancytopenia despite multi-agent treatment. In case 2, isolated high-titer de novo HLA class II antibodies were detected in association with late-onset thrombocytopenia, refractory to corticosteroids and rituximab. In both patients, plasma cell–directed therapy with daratumumab led to definitive clearance of de novo HLA antibodies and was followed by rapid and sustained hematologic recovery, including coordinated trilineage recovery in case 1 and selective platelet recovery in case 2. These cases provide initial hypothesis-generating observations that de novo HLA antibodies are associated with and may contribute to refractory IMCs after allo-HSCT. The concordance across antibody class, lineage-specific or multilineage hematopoietic injury, and lineage-matched recovery after antibody clearance suggests a functional immunophenotype–hematopoietic phenotype coupling that warrants further investigation.</p>