It has been observed that children who live on farms or in rural areas, who belong to families with three or more siblings, and who grow up in less-developed countries often have a lower incidence of allergies than other children. This rising phenomenon has been labeled the “hygiene hypothesis.”
The hygiene hypothesis proposes that exposure to allergens in the environment during a person’s early years in life minimizes the risk of developing allergies by boosting the immune system’s activity. In contrast, people who live in clean environments in early life and who lack exposure to infectious agents, symbiotic organisms, and parasites during childhood are susceptible to allergic diseases. The lack of exposure is thought to be connected to delayed development of immune tolerance.
The validity of the hygiene hypothesis is supported by clinical trials and experimental models. For example, researchers at Brigham and Women’s Hospital conducted a study on the immune system of mice living in areas that were low on bacteria and other microbes and compared them to mice living in a normal environment that contained microbes. They discovered that germ-free mice had enlarged lung and colon inflammation that resembled asthma and colitis respectively. Most importantly, the researchers found that exposing the germ-free mice to bacteria during their first weeks of life led to a normalized immune system and disease prevention. Moreover, early-life exposure to microbes provided long-lasting protection as predicted by the hygiene hypothesis.
The hygiene hypothesis, however, cannot fully explain the reason behind higher rates of allergic asthma among poor African Americans located in inner city areas. Further studies are being conducted in order to understand the role different environments play in the development of allergy.