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HLA alloimmunization is a potential complication of red blood cell (RBC) transfusion with detrimental consequences for future organ or hematopoietic stem cell transplantation.The study aimed to determine the prevalence and specificity of HLA antibodies among pediatric patients with thalassemia major (TM) and antibody changes over time while on leukoreduced chronic transfusion therapy. HLA antibodies were measured at two or more time points in children and young adults ages 3 to 21 years with TM. HLA Class I and II antibodies were measured by FlowPRA screening. Positive screening assays were confirmed with LabScreen single-antigen bead assays for antibody specificity.HLA antibodies were detected in 10 of 19 (53%) subjects: seven of 19 (37%) with HLA Class I and II antibodies, two of 19 (11%) with only HLA Class I antibodies, and one of 19 (5%) with only HLA Class II antibodies. Subjects with HLA antibodies were older (14.6 years vs. 7.1 years, p = 0.05), predominantly male (80%), and more likely to have a remote history of nonleukoreduced transfusions (p = 0.057). Median time between testing was 3.7 years. De novo HLA antibodies were detected in two of 11 patients who initially had negative panel-reactive antibody screens, while one subject lost detection of Class II antibody. Two of seven patients with HLA antibodies had antibodies to self-HLA.HLA antibodies have a high prevalence in TM patients and may be associated with nonleukoreduced transfusions and older age. For such patients, antibody identification will be useful if subsequent organ or stem cell transplantation is needed.