Background The contribution of IL‐9 to human atopy is supported by genetic studies. However, IL‐9 production in response to allergen in vitro has been reported only in children.

Objective Study IL‐9 induction by allergen in adults, compare it with IL‐5 and IL‐13 and evaluate its association with atopy.

Methods Peripheral blood mononuclear cell (PBMC) from control adults and from atopic patients were cultured with various allergens or phytohaemagglutinin (PHA) and secreted IL‐5, IL‐9 and IL‐13 were measured by ELISA.

Results IL‐9 was produced in response to Dermatophagoides pteronyssinus (Der p) by PBMC from Der p‐hypersensitive adults at levels equivalent to those induced by PHA but with slower kinetics. The induction of IL‐9 was allergen specific, reflecting donor RAST profile. In Der p‐triggered reactions of non‐atopic and atopic subjects, IL‐9 showed the highest selectivity for atopics, IL‐5 and IL‐13 being produced more frequently in non‐atopic donors. Significant correlations with specific IgE titres were found for IL‐9 with all allergens tested (Der p and two peptides of Bet v 1 birch allergen). For IL‐5 and IL‐13, they were in the same range for Der p but more variable for birch allergens. Patterns of cytokine production by individual patients in response to allergen reflected these differences: for Der p, IL‐5, IL‐9 and IL‐13 productions were strongly correlated but for birch IL‐5 differed from the latter two. The in vitro production of IL‐9 reflected clinical hypersensitivity profiles and was higher in individuals with asthma than in those with disease limited to rhinitis and/or conjunctivitis.

Conclusions Allergen‐triggered IL‐9 production in vitro is an excellent marker for atopy in adults given its virtual absence in allergen‐stimulated PBMC from non‐atopic individuals and its correlation with allergen‐specific IgE and asthma.