The tropics, helminth infections and hygiene hypotheses.

The Tropics is very appropriate to test the hypotheses raised to explain the increasing trends of allergy and other inflammatory diseases worldwide. The absence of parasite infection as a possible cause of the increase of allergic diseases was proposed by J. Gerrard a long time ago; however, the idea that helminth infections, which induce a strong Th2 could reduce allergy symptoms seems counterintuitive; but the fact is that they have a dual effect: they increase the Th2 responses but also exert immunosuppression and both effects influence the symptoms of allergy. Basic experimentation has provided valuable information about the mechanisms of allergic inflammation and more recently, about its control by helminth induced immunomodulation, discovering helminth molecules with anti-inflammatory properties that are meant to replace the live helminth therapeutic approaches. The immunosuppressive power of helminths makes them excellent candidates to be considered in the hygiene hypotheses. Future comprehensive studies evaluating simultaneously the role of microbial infections, helminth infections, microbiota, pollution and biodiversity will help to elucidate the causes of the increasing trends of allergic disorders. Doing this in the tropics, where all these variables are still present could be difficult but no doubt that will be more informative.