The hygiene hypothesis and the increasing prevalence of chronic inflammatory disorders.

The 'Hygiene' or 'Old Friends' hypothesis suggests that increases in chronic inflammatory disorders (allergies, inflammatory bowel disease and autoimmunity) in developed countries are partly attributable to diminishing exposure to organisms that were part of mammalian evolutionary history. Crucial organisms, including helminths and saprophytic mycobacteria, are recognised by the innate immune system as harmless or, in the case of helminths, as organisms that once established must be tolerated. This recognition then triggers development of regulatory dendritic cells that drive regulatory T-cell responses to the 'Old Friends' themselves and to simultaneously processed 'forbidden' target antigens of the chronic inflammatory disorders.