We show that a single treatment with IL-33 and TL1A is sufficient for induction of IL-9-producing ILC2s in vivo and that endogenous IL-33 and TL1A are necessary for rapid induction of IL-9high ILC2s after a single exposure of naïve mice to A. alternata, a major asthma-associated allergen. The gene discussed is IL33; the disease is asthma.