The "hygiene hypothesis" of allergy and autoimmune disorders

This abstract from Clin Exp Immuol provides a concise summary:

According to the ‘hygiene hypothesis’, the decreasing incidence of infections in western countries and more recently in developing countries is at the origin of the increasing incidence of both autoimmune and allergic diseases. The hygiene hypothesis is based upon epidemiological data, particularly migration studies, showing that subjects migrating from a low-incidence to a high-incidence country acquire the immune disorders with a high incidence at the first generation. However, these data and others showing a correlation between high disease incidence and high socio-economic level do not prove a causal link between infections and immune disorders. Proof of principle of the hygiene hypothesis is brought by animal models and to a lesser degree by intervention trials in humans. Underlying mechanisms are multiple and complex. They include decreased consumption of homeostatic factors and immunoregulation, involving various regulatory T cell subsets and Toll-like receptor stimulation. These mechanisms could originate, to some extent, from changes in microbiota caused by changes in lifestyle, particularly in inflammatory bowel diseases. Taken together, these data open new therapeutic perspectives in the prevention of autoimmune and allergic diseases.

Some other excerpts from the publication:

The hypothesis was first proposed by Strachan, who observed an inverse correlation between hay fever and the number of older siblings when following more than 17 000 British children born in 1958... The leading idea is that some infectious agents – notably those that co-evolved with us – are able to protect against a large spectrum of immune-related disorders...

In 1998, about one in five children in industrialized countries suffered from allergic diseases such as asthma, allergic rhinitis or atopic dermatitis. This proportion has tended to increase over the last 10 years, asthma becoming an ‘epidemic’ phenomenon... The prevalence of atopic dermatitis has doubled or tripled in industrialized countries during the past three decades, affecting 15–30% of children and 2–10% of adults... Part of the increased incidence of these diseases may be attributed to better diagnosis or improved access to medical facilities in economically developed countries. However, this cannot explain the marked increase in immunological disorder prevalence that has occurred over such a short period of time in those countries, particularly for diseases which can be diagnosed easily...