A recent study on children attending day care or nurseries by de Jongste in the American Thoracic Society journal cast some doubt on the so-called Hygiene Hypothesis for allergy development. The Hygiene Hypothesis notion that farm animal faeces exposure and childhood infections will prevent allergies has been promoted for decades. The hygiene hypothesis essentially links a more clean and sterile home environment with the overall rise in allergies seen in many developed Western counties. Poor living conditions with early exposure to germs, infections and parasites seem to shift the infant’s immune system into survival mode (TH1) and away from allergy mode (TH2) when allergy testing. However this immune switching probably occurs very early in the first few months of life. Therefore as mentioned in the American study, day care centre exposure and subsequent childhood infections may have little impact on allergy development. Particularly if the child attends a day-centre after one year of age when their immune type reactivity is more established. It still seems likely that a germ-filled household with difficult living conditions and plenty of sickly older siblings will be more protective from allergies while a sterile, insular environment in early infancy seems to promote allergies.
http://news.bbc.co.uk/2/hi/health/8241774.stm
