Dendritic cells can prime naive CD4⁺ T cells; however, here we demonstrate that dendritic cell–mediated priming was insufficient for the development of T helper type 2 cell–dependent immunity. We identify basophils as a dominant cell population that coexpressed major histocompatibility complex class II and interleukin 4 message after helminth infection. Basophilia was promoted by thymic stromal lymphopoietin, and depletion of basophils impaired immunity to helminth infection. Basophils promoted antigen-specific CD4⁺ T cell proliferation and interleukin 4 production in vitro, and transfer of basophils augmented the population expansion of helminth-responsive CD4⁺ T cells in vivo. Collectively, our studies suggest that major histocompatibility complex class II–dependent interactions between basophils and CD4⁺ T cells promote T helper type 2 cytokine responses and immunity to helminth infection.