Immunity to the intestinal parasite Heligomosomoides polygyrus is dependent on the successful generation of T helper 2 (Th2) memory cells. We showed that B cells contribute to immunity against H. polygyrus by producing antibody (Ab) and by promoting expansion and differentiation of primary and memory Th2 cells. We also demonstrated that cytokine-producing effector B cells were essential for effective immunity to H. polygyrus. Tumor necrosis factor alpha production by B cells was necessary for sustained Ab production, whereas interleukin 2 production by B cells was necessary for Th2 cell expansion and differentiation. These results show that B cells mediate protection from pathogens not only by presenting antigen and secreting antibody but also by producing cytokines that regulate the quality and magnitude of humoral and cellular immune responses.