Epidemiological and experimental studies have shown that exposure to the gastric bacterium Helicobacter pylori, especially in early life, prevents the development of asthma. Recent mouse studies have shown that this protective effect does not require live bacteria and that treatment with an extract of H. pylori in neonates prevents the development of airway inflammation and goblet cell metaplasia. In the current study, the effect of administration of an extract of H. pylori was assessed in a therapeutic study design with application of the extract just prior to allergen challenge. C57BL/6 mice were sensitized and challenged with OVA or house dust mite. Treatment with H. pylori extract just prior to the challenge significantly reduced airway inflammation, as assessed in bronchoalveolar lavage fluid and lung tissue, and reduced airway remodeling, as assessed by goblet cell quantification. These effects were apparent in the OVA model and in the house dust mite model. Injection of H. pylori extract reduced the processing of allergen by dendritic cells in the lungs and mediastinal lymph node. Bone marrow-derived dendritic cells exposed to H. pylori extract were affected with regard to their ability to process Ag. These data show that application of H. pylori extract after sensitization effectively inhibits allergic airway disease.