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Helminth infections are characteristically associated with Th2-dependent responses leading to IgE production and eosinophilia. However, the effect of such responses varies between different hosts. In murine schistosomiasis, for example, Th1 responses are associated with protection against infection, and Th2 responses with egg-induced morbidity. In human schistosomiasis, in contrast, Th2 responses with the production of IgE antibodies against a restricted range of adult worm antigens are associated with protection against reinfection after chemotherapy, while individuals in whom Th2 responses against egg antigens predominate show less severe egg-associated morbidity than those in whom Th1 responses predominate. It is suggested that Th2-mediated responses are selectively advantageous to the human host in the context of schistosomiasis and possibly other helminth infections. The implications of this for vaccination, for control through chemotherapy, and for the study of interactions with other infections are briefly discussed.