Thus IFN-γ/IRF1 signalling restricts Th9-mediated airway allergy in an IL-9-dependent manner while enhancing IFN-γ production in Th9 cells, indicating that IRF1 constrains allergic airway disease by skewing Th9 cells towards a ‘low-allergic' IFN/Th9 phenotype. Here, IFNA1 is linked to allergic respiratory disease.