The factors that contribute to allergic asthma are unclear but the resulting condition is considered a consequence of a type-2 T helper (T(H)2) cell response. In a model of pulmonary allergic inflammation, mice that lacked gamma delta T cells had decreases in specific immunoglobulin E (IgE) and IgG1 and pulmonary interleukin-5 (IL-5) release as well as in eosinophil and T cell infiltration compared with wild-type mice. These responses were restored by administration of IL-4 to gamma delta T cell-deficient mice during the primary immunization. Thus, gamma delta T cells are essential for inducing IL-4-dependent IgE and IgG1 responses and for T(H)2-mediated airway inflammation to peptidic antigens.