Parasitic infection and immunomodulation: A possible explanation for the hygiene hypothesis in autoimmune and allergic disease

Helminthic parasites have a long history of co-evolution with human beings. The incidence of helminthic infection has significantly decreased in developed countries due to better sanitary measures. However, epidemiological data suggest a corresponding increase in the incidence of autoimmune and allergic diseases in association with a reduction in helminthic infections in these societies. The immune response to helminthic infection involves both innate and adaptive processes, with a strongly polarised Th2 response being the most characteristic feature. However, there is a concomitant increase in the functional regulatory T cell responses. This might explain the paradoxical decrease in both Th2-and Th1-mediated diseases such as allergy and immune-mediated inflammatory disorders in populations with increased incidence of helminthic infection. Parasitic infection therefore appears to confer a degree of immunomodulation, and for this reason, utilising helminthic infection as a therapeutic modality for the treatment of allergic and autoimmune disease has been proposed. Improved understanding of the immunologic responses to helminth infection allows these mechanisms to be exploited, enabling manipulation of the immune response in Th1-dominant conditions such as inflammatory bowel disease and multiple sclerosis, and providing a new approach to treatment of these and other inflammatory and allergic conditions.