Contributions by basophils to allergic and helminth immunity remain incompletely defined. Using sensitive interleukin 4 (Il4) reporter alleles, we demonstrate here that basophil IL-4 production occurs by a CD4(+) T cell-dependent process restricted to the peripheral tissues affected. We genetically marked and achieved specific deletion of basophils and found that basophils did not mediate T helper type 2 (T(H)2) priming in vivo. Two-photon imaging confirmed that basophils did not interact with antigen-specific T cells in lymph nodes but engaged in prolonged serial interactions with T cells in lung tissues. Although targeted deletion of IL-4 and IL-13 in either CD4(+) T cells or basophils had a minimal effect on worm clearance, deletion from both lineages demonstrated a nonredundant role for basophil cytokines in primary helminth immunity.