en Molecular Immunology & Vaccines Molecular Immunology & Vaccines مقاله کامل Full Length/Original Article <p><strong>Background: </strong>Allogenic hematopoietic stem cell transplantation (HSCT) is a strategy used for treatment of different malignant diseases. However, success of allo-HSCT can be hampered by graft-versus-host-disease (GVHD). Natural killer (NK) cells may play an important role in activating antigen presenting cells and subsequent activation of T cells. The main purpose of this study was the evaluation of IL-21, as a blood biomarker, for early detection of acute GVHD (aGVHD) in children after HSCT and also the study of human leukocytes antigen (HLA)-C1 polymorphism, as a targeting ligand for NK cells in these patients. <strong>Methods</strong>: Fifty one children receiving HSCT were studied. Blood samples were collected at -8, 7, and 14 days of transplantation. The -8-day samples were analyzed for HLA-C1 polymorphism by PCR-sequence-specific primer technique and pre-transplantation IL-21 assay. To study the serum levels of IL-21, two blood samples were collected on days +7 and +14 and analyzed by ELISA technique. <strong>Results</strong>: The results indicated that the incidence of aGVHD in pediatric is associated with a polymorphism of HLA-C1, as alleles HLA-C01:12 (<em>P</em><0.001), HLA-C01:22 (<em>P</em><0.004), and HLA-C01:67 (<em>P</em><0.009). On the other hand, the serum levels of IL-21 in children with aGVHD were decreased after transplantation compared to before transplantation. The serum levels of the IL-21 at 14 days after transplantation had a significant correlation with the occurrence of aGVHD (<em>P</em>=0.05).<strong> Conclusion</strong>: Based on the findings of this study, there is a significant correlation between HLA-C1 polymorphisms and the serum levels of IL-21 with the incidence of aGVHD.</p> Hematopoietic stem cell transplantation, NK cells, IL-21 392 399 http://ibj.pasteur.ac.ir/browse.php?a_code=A-10-1-670&slc_lang=en&sid=1 Nasrin Sehati No Student Research Committee, Semnan University of Medical Sciences, Semnan, Iran Parviz Kokhaei Yes Cancer Research Center and Department of Immunology, Semnan University of Medical Sciences, Semnan, Iran Ali Motevalizade Ardekani No National Institute of Genetic Engineering and Biotechnology (NIGEB), Tehran, Iran Raziyeh Tootoonchian No Student Research Committee, Semnan University of Medical Sciences, Semnan, Iran Fatemeh Pak No Cancer Research Center and Department of Immunology, Semnan University of Medical Sciences, Semnan, Iran