Recurrent airway obstruction is characterized by more severe airway obstruction and marked airway neutrophilia, in comparison with the mild mixed airway inflammation usually present in IAD.29, 30 Neutrophilia predominates in the allergic disease process in horses partially because of IL‐4‐mediated release of neutrophilic chemotactic factors,31 and the severity of disease correlates to some extent with the severity of airway neutrophilia.32 This evidence concerns the gene IL4 and allergic disease.