The pathophysiological roles of neurotrophins in allergic asthma are as follows: first, elevated neurotrophin levels in bronchoalveolar fluid (BALF); second, blood neurotrophin levels correlate with airflow limitation; third, the induction of airway hyperactivity and airway obstruction through the production of ROS- and MAPK-mediated allergen-induced airway inflammation, modulation of neurite formation and cellular contractility, and proinflammatory cytokine release; finally, airway smooth muscle proliferation and matrix metalloproteinase induction (137). This evidence concerns the gene BDNF and allergic asthma.