Both metazoan parasites and simple protein allergens induce T helper type 2 (T_H2) immune responses, but the mechanisms by which the innate immune system senses these stimuli are unknown. In addition, the cellular source of cytokines that control T_H2 differentiation in vivo has not been defined. Here we showed that basophils were activated and recruited to the draining lymph nodes specifically in response to T_H2–inducing allergen challenge. Furthermore, we demonstrate that the basophil was the accessory cell type required for T_H2 induction in response to protease allergens. Finally, we show that basophils were directly activated by protease allergens and produced T_H2-inducing cytokines, including interleukin 4 and thymic stromal lymphopoietin, which are involved in T_H2 differentiation in vivo.