TLR4 and airway hyperresponsiveness: The airway epithelium, through secreted cytokines that include IL-25 and thymic stromal lymphopoietin (TSLP) and endogenous proteinases such as matrix metalloproteinase 7 (MMP7), and novel proteinase-activated inflammatory pathways that include fibrinogen and Toll like receptor 4 (TLR4), further promote airway hyperresponsiveness and allergic inflammation [21–25].