Allergic diseases are characterized by overactive type 2 immune responses to allergens and immunoglobulin E (IgE)-mediated hypersensitivity. Emerging evidence suggests that follicular helper T (T ) cells, rather than type 2 T-helper (T 2) cells, play a crucial role in controlling IgE production. However, follicular regulatory T (T ) cells, a specialized subset of regulatory T (T ) cells resident in B-cell follicles, restricts T cell-mediated help in extrafollicular antibody production, germinal center (GC) formation, immunoglobulin affinity maturation, and long-lived, high-affinity plasma and memory B-cell differentiation. In mouse models of allergic asthma and food allergy, CXCR5 T cells, not CXCR5 conventional T 2 cells, are needed to support IgE production, otherwise exacerbated by CXCR5 T cell deletion. Upregulation of T cell activities, including a skewing toward type 2 T (T 2) and IL-13 producing T (T 13) phenotypes, and defects in T cells have been identified in patients with allergic diseases. Allergen immunotherapy (AIT) reinstates the balance between T and T cells in patients with allergic diseases, resulting in clinical benefits. Collectively, further understanding of T and T cells and their role in the immunopathogenesis of allergic diseases creates opportunities to develop novel therapeutic approaches.
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