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Human schistosomiasis decreases immune responses to allergens and clinical manifestations of asthma.

Studies have demonstrated that people living in areas endemic for helminths have a decreased reactivity to skin prick tests to aeroallergens and milder forms of asthma. Hypotheses to explain the inverse correlation between helminth infections and atopy include competition between helminth-induced polyclonal IgE and aeroallergen-specific IgE for high-affinity receptors present on mast cells, increased number of regulatory T cells, and high levels of regulatory cytokines, such as IL-10, produced during helminthic infections. Indeed, cells from asthmatic individuals infected with Schistosoma mansoni produce lower levels of IL-5 than asthmatics free of infections. In contrast, IL-10 is more readily produced by allergen-stimulated cells from asthmatics who are infected and is detected only at low levels by cells from helminth-free asthmatics. It is well known that Th2 cytokines are involved in the pathogenesis of allergies and asthma, and some studies indicate that IL-10 is the key cytokine that inhibits the Th2-inflammatory response in allergy. In this chapter we will discuss the association between S. mansoni infection, atopy and severity of asthma and possible mechanisms by which individuals living in helminth endemic areas are protected against the development of allergies.