There are two hypotheses for the primary function of the TH/IgE system: protection against helminths that 'spills over' into inappropriate allergic responses in the modern environment, or protection from a variety of environmental carcinogens and infectious diseases that is adaptive even today. We suggest that rather than being alternatives, these two hypotheses fit into a single causal framework. Atopic responses to helminths kill, inhibit, or expel the parasites, thereby reducing their debilitating effects, the most serious of which is bladder cancer. The ultimate (evolutionary) reason for the TH2/IgE system may be minimizing chances that multiple biotic and abiotic hazards and carcinogens will enter the body; in some tropical areas, helminths are the major proximate (immediate) triggers of this response.
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