NPS and asthma: Three main mechanisms have been proposed to cause airway inflammation in asthma: type I allergic reaction dominated by Th2 cytology, i.e., immunogenic inflammatory theory; epithelial injury and reactivation of epithelial–mesenchymal trophic units dominated by transfer growth factor, endothelin-1, and metalloproteinase; and neuropeptide-induced neurogenic inflammatory theory; however, these sources of inflammation cannot fully explain the pathogenesis of various types of asthma (Rogers, 2001).