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The Hygiene Hypothesis: Can We Be Too Clean?

By — American Academy of Allergy, Asthma, & Immunology
Updated on Mar 10, 2009

Many parents and physicians are observing a worrying trend: more and more children are suffering from food allergies, eczema, asthma, hay fever and other allergic maladies. Why are allergies becoming more frequent when most other childhood disorders are decreasing and our medical treatment options keep improving?

The “hygiene hypothesis” claims that lack of stimulation of our immune systems with bacteria or viruses could lead to higher rates of allergies and asthma. While this approach can not completely explain the rise in allergic and autoimmune diseases, it does offer some explanation for the shocking increases.

To understand the hygiene hypothesis, it is important to know how allergies develop. Allergies are caused by unusual immune responses to things we usually encounter and tolerate without problems, such as animal dander, dust or pollen.

The main function of the immune system is to defend us from dangerous bacteria and viruses. When your child comes in contact with these pathogens, the immune system goes to work fighting them off. But in allergic people, the immune system attacks harmless substances.

The first time your child comes into contact with the allergen, the body’s immune system sends out signals to attack the foreign invader. Often, there is no allergic reaction following the first exposure, but the body becomes “sensitized.” The next time the allergen is encountered, the body responds fast – usually within 30 minutes – and the reaction can be mild or severe.

Professor David Strachan developed the hygiene hypothesis in 1989 while trying to explain the increase in allergic diseases since industrialization, and the higher incidence of allergic diseases in more developed countries. He was the first to publish an articledescribing his observation that allergic diseases, such as hay fever and eczema, were less common in children who came from larger families. He concluded that those children may be exposed to more diseases, bacteria and infections through contact with their many siblings, helping to build their immune systems. Children without siblings might not receive the same exposure.

That’s not the only evidence to support a hygiene hypothesis. Studies have also shown that various immunological and autoimmune diseases are much less common in the developing world than the industrialized world. Interestingly, immigrants who come to the West are likely to develop immunological disorders.

Other studies indicate that immune system stimulation begins in the first moments of life. Birth via cesarean section, rather than vaginal birth, has been associated with a higher risk of developing allergies later in life. Exposure to bacteria in the birth canal can stimulate the infant’s immune system early on.

Does this theory mean that vaccines are bad for our kids?
Actually, vaccinations are important to prevent serious consequences (including death) from dangerous childhood diseases. The majority of studies examining the influence on vaccination of development of allergies have not found an increased risk for allergies or asthma in vaccinated children. In fact, some studies have shown exactly the opposite: vaccinations can actually protect against allergies. However, the majority of studies refute this result, and if there is any effect, it’s probably a weak one.
Is it good for kids to be exposed to bacteria and viruses to prevent allergies?
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