The Hygiene Hypothesis: Epidemiologic Evidence

Several epidemiologic studies indicate that the increased prevalence of allergic diseases is associated with a reduced microbial stimulation of the immune system, thus providing a consistent evidence-based support to the hypothesis proposed by Strachan that hygienic conditions represent a risk factor for development of allergy (hygiene hypothesis). Personal data in this expanding area of research seem to confirm the hygiene hypothesis in both European (1) and American (2) large population samples. However, they also suggest that the time and type of infection represent crucial factors in determining the susceptibility for allergic diseases that are also influenced by other genetic and environmental factors related to population sample and style of life considered. It has been suggested that microbial stimulation or hygiene influence the susceptibility of allergic diseases by modulating the Th1/Th2 balance (3). However, since the reduced prevalence of infectious diseases seems to be associated not only with an increased prevalence of Th2 allergic diseases but also with an increased prevalence of typical Th1-mediated diseases (4), the Th1-Th2 hypothesis seems too simplistic to completely explain the hygiene hypothesis. Additional factors such as defects in regulatory T cell (Treg) number or function probably play a relevant role in modulating the complex immune response to a combined microbial, allergenic and autoantigenic stimulation. Certainly, the studies of the relationships between infections and allergic-immunologic diseases should open new perspectives in the approaches to treatment of these frequent, chronic and invalidating conditions (Fig. 1)(5).FIG. 1: Schematic representation of the putative microbial and immunologic events underlying the hygiene hypothesis.Key Questions 1) What kind of epidemiologic studies should be performed to provide further understanding and support for the hygiene hypothesis? 2) Which experimental studies should be triggered by epidemiologic studies? 3) Is there enough background information to indicate the use of bacterial products in humans to prevent allergies?