Hygiene, microbial diversity and immune regulation.

Purpose of review Sanitation, antibiotics and vaccines have done more to extend life expectancy than any other medical innovation. Concern exists, however, about the link between hygiene and increased incidence of immune-mediated disorders. Recent findings Studies confirm higher prevalence of asthma and allergic diseases in urban areas and developed countries rather than rural areas and developing countries. There is an inverse association between family size and disease. The debate over whether infection precipitates or prevents immune dysregulation remains a contentious one. Our knowledge about the microbial composition of the intestinal ecosystem is expanding rapidly with the introduction of molecular techniques. Differences in gut bacteria between health and atopy or inflammatory bowel disease are repeatedly reported. Recent data in inflammatory bowel disease suggest reduced species diversity and temporal instability of the gut microecosystem. The gut is a major site for induction of regulatory T cells, which secrete immunosuppressive cytokines. Not only infections, but also some commensals induce regulatory pathways, which seem to be functionally deficient in multiple sclerosis, allergies and inflammatory bowel disease. Changes in lifestyle leading to decreased exposure to certain nonpathogenic species that are important for the development of immunoregulatory mechanisms are probably associated with increased incidence of some immune-mediated diseases, such as allergies and inflammatory bowel disease.