Parasitic infection: Good or bad for the hygiene hypothesis?

Abstract

Background: The increase in prevalence of allergic disorders in industrialized countries has been attributed to changes in lifestyle and the environment. Such changes have led, among others, to a lower exposure to microbes/microbial products. The hygiene hypothesis proposes that less frequent exchange of microbes may alter the immune system in such a way that upon encounter of an innocuous antigen, such as an aeroallergen, t-helper cell type 2 (Th2) responses are readily induced, which lead to allergic disease. The hygiene hypothesis has remained controversial due to disagreement between epidemiological studies where exposure to micoorganisms is not always associated with lower prevalence of allergies. Method/Data base: Here we review the common immunological pathways and the association between helminth infections and allergic diseases. Results/Conclusion: The surprising observations that helmith infections can either be positively or negatively associated with allergies may be explained by the differences in parasite burden and chronicity or parasite species. Prospective randomized double-blind placebo-controlled anti-helminth trials are needed to assess the complex relationship between parasitic helminths and allergic disorders. © 2005 Hogrefe & Huber Publishers.