Figure 1.

Schematic representation of pathogenic processes in allergic disease. The hallmark of Type I allergic immune response is production of allergen-specific IgE and activation of mast cells, basophils and eosinophils. Recent literature suggests that T_H2 cytokines are induced after allergen exposure, even before the establishment of an allergen-specific adaptive immune response. Thus both innate immunity and adaptive immunity are instrumental in establishing Type I allergic immune response. The upper strata of the figure depicts that upon allergen exposure in a suitable environmental milieu, IL-25, IL-33 and TSLP are produced by epithelial cells. For the innate arm, these cytokines act on innate lymphoid/myeloid type 2 cells that are capable of producing large amounts of T_H2 cytokines, such as IL-4, IL-5, IL-9 and IL-13. For the adaptive arm, allergens are taken up and processed by TSLP and IL-33 primed DCs and presented to naïve allergen-specific T cells in a T_H2 milieu that drives their differentiation into T_H2 cells. Dominant T_H2 responses to allergens during the sensitization phase lead to immunoglobulin class switching to IgE as well as recruitment and activation of pro-inflammatory cells such as mast cells, basophils and eosinophils.