T follicular helper (Tfh) cells in normal immune responses and in allergic disorders

Follicular helper T cells (T-fh) are located within germinal centers of lymph nodes. Cognate interaction between T-fh, B cells, and IL-21 drives B cells to proliferate and differentiate into plasma cells thereby leading to antibody production. T-fh cells and IL-21 are involved in infectious and autoimmune diseases, immunodeficiencies, vaccination, and cancer. Human peripheral blood CXCR5(+) CD4(+) T cells comprise different subsets of T-fh-like cells. Despite the importance of the IgE response in the pathogenesis of allergic disorders, little is known about the role of follicular and blood T-fh cells and IL-21 in human and experimental allergic disease. Here, we review recent advances regarding the phenotypic and functional characteristics of both follicular and blood T-fh cells and of the IL-21/IL-21R system in the context of allergic disorders.