TSLP and viral respiratory tract infection: Th2 type inflammation in the airways often starts in childhood, when environmental stimuli such as viral respiratory tract infection, exposure to parental smoking, and NO2 and other airborne pollutants or allergens activate airway epithelial cells to produce type 2 inflammatory cytokines including IL-25, IL-33, or TSLP (thymic stromal lymphopoietin) (Fig. 4) [56].