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Background: The function of follicular regulatory T (T FR ) cells, especially in regulating IgE production in patients with allergic diseases, is poorly understood., Objective: We sought to investigate the phenotype, function, and clinical relevance of T FR cells in patients with allergic rhinitis (AR)., Methods: The phenotype and frequency of tonsillar and circulating T FR cells were characterized by using flow cytometry. T FR cell function was examined in an assay by coculturing with follicular helper T cells and B cells. The associations between T FR cells and the clinical features in patients with AR before and after allergen immunotherapy (AIT) were analyzed., Results: T FR cells were detected in germinal centers of tonsils, but compared with subjects without AR, the frequencies decreased in patients with AR who were allergic to house dust mites. Circulating T FR cells in blood were phenotypically and numerically correlated with tonsillar T FR cells, and a reduction of circulating T FR cells but not total or CXCR5 - regulatory T cells was noted in patients with AR compared with healthy control subjects. Moreover, circulating T FR cells in patients with AR showed a specific defect in suppressing IgE production but were capable of suppressing production of other immunoglobulin types. We identified negative associations of circulating T FR cell frequencies and function with antigen-specific IgE levels or disease severity in patients with AR. After AIT, the frequencies and function of circulating T FR cells were improved, which positively associated with disease remission., Conclusion: Impairment in T FR cells might contribute to aberrant IgE production in patients with AR, and AIT improves defective T FR cell function. T FR cells might serve as a potential biomarker to monitor clinical response to AIT., (Copyright © 2019 American Academy of Allergy, Asthma & Immunology. Published by Elsevier Inc. All rights reserved.)