Allergic responses are characterised by the activation of a specific subset of effector CD4 T cells, the Th2 cells, that respond to harmless environmental antigens causing inflammation and pathology. Th2 cells are also found in the context of parasite infections, where they can mediate parasite clearance and expulsion, and support tissue repair. The process that leads to the activation of Th2 cells in vivo is incompletely understood: while it has become clear that ‘conventional’ dendritic cells are essential antigen presenting cells for the initiation of Th2 immune responses, the molecules that are expressed by dendritic cells exposed to allergens, and the mediators that are produced as a consequence, and signal to naïve CD4 T cells to promote their development into effector Th2, remain to be defined. Here we summarise recent developments in the identification of the dendritic cell subsets involved in Th2 responses, review potential mechanisms proposed to explain the generation of these immune responses, and the direct and indirect signals that condition dendritic cells during allergen or parasite exposure to drive the development of Th2 responses.
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