Helminth infections and allergic diseases: from the Th2 paradigm to regulatory networks.

Both helminth infections and allergic diseases are characterized by a Th2 response. The former mostly affects developing countries while the later is steadily increasing in westernized countries, since most parasitic diseases have been eradicated. However, besides the cytokine pattern, both pathologies display striking similarities regarding the cell types involved in the process, the mucosal component of the inflammatory reaction and the immunoglobulin isotypes produced, IgE and IgA. Recent advances also suggest that allergic pathologies and some other inflammatory diseases arise from a similar dysfunction of the regulatory mechanisms, existing within the immune system. Such a dysregulation leads to a rupture of tolerance towards allergen. Chronic and moderate helminth infections might be beneficial to the induction of various regulatory mechanisms, involving specific regulatory cell populations (dendritic cells and lymphocytes), inhibitory receptors, blocking antibodies and two prominent cytokines: IL-10 and TGF-b, which would decrease the incidence of allergic diseases. This mechanism could provide a new vision of the so-called "hygiene hypothesis".