Engraftment across major ABO blood group and rhesus E antigen incompatibility in the presence of persistent anti-E antibodies and late serologic findings coinciding with disease relapse.

ABO incompatibility is frequent in haematopoietic stem cell transplantation (HSCT), yet the clinical impact of non-ABO red blood cell (RBC) alloimmunisation remains less defined. We describe the clinical course of a man in his 50s with chronic eosinophilic leukaemia who underwent a mismatched unrelated donor (MMUD) HSCT. Despite a major ABO mismatch and high-titre pre-existing anti-E antibodies against an E-antigen positive donor, the patient achieved successful trilineage engraftment without haemolysis. Six months after the transplant, he developed severe pancytopenia and a positive direct antiglobulin test (DAT) with anti-E elution. These serologic findings coincided with disease relapse rather than isolated immune-mediated haemolysis. This case illustrates that successful engraftment is possible across RhE-incompatibility and underscores the need to interpret late serologic abnormalities within the broader context of graft function and disease status.