# Case Report: Allogeneic hematopoietic stem cell transplantation in a patient with triple-allele expression at both HLA-B and HLA-C loci

## TL;DR

A rare case of a patient with three HLA alleles at two loci led to a unique donor selection process for a successful stem cell transplant.

## Contribution

Reports a rare case of triple-allele HLA expression influencing donor selection in allogeneic stem cell transplantation.

## Key findings

- A patient had three alleles at both HLA-B and HLA-C loci, inherited from a maternal haplotype.
- All three alleles were expressed on the cell surface, confirmed by flow cytometry.
- A sibling donor sharing the same triple-allele haplotype was successfully used despite an HLA-A mismatch.

## Abstract

Human leukocyte antigen (HLA) matching is critical for donor selection in allogeneic hematopoietic stem cell transplantation (allo-HSCT). Each HLA locus is normally biallelic; however, exceptionally rare cases with more than two alleles at a single locus have been reported.

We describe a 17-year-old male with mixed phenotype acute leukemia (T/myeloid) who was found to carry three alleles at both the HLA-B and HLA-C loci during pre-transplant evaluation. Family-based HLA typing revealed that the additional alleles were inherited as part of a single maternal haplotype shared by multiple siblings. Flow cytometric analysis using antisera with known HLA specificity demonstrated that all three HLA-B and HLA-C alleles were expressed on the cell surface. Because this unusual immunogenetic configuration precluded the identification of an unrelated donor, a sibling donor sharing the same triple-allele haplotype was selected despite a single HLA-A mismatch. Allo-HSCT was successfully performed, with manageable graft-versus-host disease.

This case highlights an extremely rare immunogenetic configuration in which triple-allele expression at two HLA class I loci directly influenced donor selection for allo-HSCT. Comprehensive interpretation of HLA typing results, including family analysis and protein-level expression, is essential when such atypical findings are encountered.