Activation of airway epithelial cells. The figure shows the various pattern recognition receptors that are expressed on epithelial cells that can bind HDM-associated or induced ligands, including β-glucans, ATP, proteases (Der p 1, 3, 6 and 9) and endotoxin. Expression of these receptors on sub-epithelial fibroblasts and smooth muscle cells and the receptors for the epithelium-derived cytokines TSLP, IL-25 and IL-33 are depicted.