How are T(H)2-type immune responses initiated and amplified?

CD4(+) T helper (T(H)) cells have crucial roles in orchestrating adaptive immune responses. T(H)2 cells control immunity to extracellular parasites and all forms of allergic inflammatory responses. Although we understand the initiation of the T(H)2-type response in tissue culture in great detail, much less is known about T(H)2 cell induction in vivo. Here we discuss the involvement of allergen- and parasite product-mediated activation of epithelial cells, basophils and dendritic cells and the functions of the cytokines interleukin-4 (IL-4), IL-25, IL-33 and thymic stromal lymphopoietin in the initiation and amplification of T(H)2-type immune responses in vivo.