Hygiene hypothesis: A hypothesis that states that exposure to allergens in the environment early in life reduces the risk of developing allergies by boosting immune system activity. Conversely, relatively clean environment in early life would sway the immune system towards allergy-promoting responses. Also called the hygiene theory.
The hygiene hypothesis may explain the rising incidence of allergic diseases and facts such as the following: -- the lower incidence of allergy in those living on farms or in rural areas (due possibly to more exposure to bacteria in barns and elsewhere in the country); the lower incidence of allergy in younger children of large families with 3 or more older siblings (due perhaps to repeated exposure to infection from older siblings); and the lower incidence of asthma and wheezing in children who go to day care centers (where they are exposed to more infections). The hygiene hypothesis, however, cannot explain the higher rates of allergic asthma among poor African Americans in the inner city areas.