The existence of the canine hookworm Ancylostoma caninum in the human gut seems to be curtailed by an effective IgE-mediated inflammatory response against the helminth's feeding secretions, which, by damaging the host, compromises the parasite's feeding opportunities. In contrast, immunity does not develop against adapted, anthropophilic hookworm species, and it is suggested this relates to modulation of the inflammatory response by hookworm secretions. Here, John Croese discusses this hypothesis and its implications for the development of drugs to suppress allergy.