Exaggerated airway constriction triggered by exposure to irritants such as allergen, also called hyperreactivity, is a hallmark of asthma and can be life-threatening. Aside from immune cells, vagal sensory neurons are important for airway hyperreactivity 1-4. However, the identity and signature of the downstream nodes of this adaptive circuit remains poorly understood. Here we show that Dbh+ neurons in the nucleus of the solitary tract (nTS) of the brainstem, and downstream neurons in the nucleus ambiguus (NA), are both necessary and sufficient for chronic allergen-induced airway hyperreactivity. We found that repeated exposures of mice to inhaled allergen activates nTS neurons in a mast cell-, interleukin 4 (IL-4)- and vagal nerve-dependent manner. Single-nucleus RNA-seq followed by RNAscope quantification of the nTS at baseline and following allergen challenges reveals that a Dbh+ population is preferentially activated. Ablation or chemogenetic inactivation of Dbh+ nTS neurons blunted, while chemogenetic activation promoted hyperreactivity. Viral tracing indicates that Dbh+ nTS neurons, capable of producing norepinephrine, project to the NA, and NA neurons are necessary and sufficient to relay allergen signals to postganglionic neurons that then directly drive airway constriction. Focusing on transmitters, delivery of norepinephrine antagonists to the NA blunted allergen-induced hyperreactivity. Together, these findings provide molecular, anatomical and functional definitions of key nodes of a canonical allergen response circuit. The knowledge opens the possibility of targeting neural modulation as an approach to control allergen-induced airway constriction. Overall design: Mice were euthanized using CO2 inhalation. For nTS, brainstems were acutely harvested from either four groups of adult wild-type naïve mice (N=4 biological repeats, 2 males in each group); from mice at 1.5 hours after the 4th saline treatment (N=1, 2 males); and from mice at 1.5 hours after the 4th HDM challenge (N=1, 2 males). nTS was visualized by microscopy and harvested based on anatomical landmarks.