Microbiology: A Systems Approach 4th Edition by Marjorie Kelly Cowan

Exercise 32

Let Them Eat Dirt
Your child is crawling around on the floor and happens upon a remnant of a cookie. Your first reaction may be to swipe that dirty cookie out of his little hands, but you may be doing the child a favor by letting him gnaw on that dusty crumb. Doctors and scientists agree-let kids get dirty. In 1989, Dr. David P. Strachan investigated the "hygiene hypothesis," which had previously been cited to explain the increased rates of allergies, eczema, and asthma seen in children living in urban areas. Other studies have found that children growing up in rural farm areas with many animals have lower incidences of asthma and allergies. This is most likely due to the fact that children growing up on a farm come from larger families, they have pets and farm animals, and their houses are heated with wood and coal, among other factors-all of which expose them to microbes and other substances. Children who grow up in relatively affluent urban areas have less exposure to animals, dust, dirt, and microbes and have higher rates of atopy, or chronic local allergy, than their rural counterparts. Dr. Strachan's research showed that hay fever, asthma, and eczema were also less common in children growing up in larger families than families with only one child.
Hay fever, asthma, hives, and other atopic allergies are all mediated by IgE antibodies and are among the most common chronic illnesses seen in children in the industrialized world. As you have learned, exposure to microbes often occurs without infection, and the constant but subtle exposure to microbes and their products is vital to the proper development of a child's immune system. If there is little exposure to materials that stimulate the developing immune system, the more likely it is that there will be an overreaction to innocuous substances such as dust, pollen, or pet dander.
How does exposure to microbes early in life affect the immune system
What cells are involved in the atopic response
Recently, researchers at Brigham and Women's Hospital compared the immune systems of "germ-free" mice-mice that had never been exposed to microbes and lived in completely sterile conditions with their normal, "dirty" counterparts. The sterile, germ-free mice had an exaggerated inflammatory response in their lungs resembling asthma and in their colon resembling irritable bowel syndrome (IBS). This out-of- control inflammatory response was caused by a class of T cells called invariant natural killer T cells (INKT cells). INKT cells release several interleukins that initiate the inflammatory response in the airways, leading to symptoms of hay fever and asthma. When germ-free mice were exposed to microbes early in life, they developed healthy, normal immune systems; but when adult germ-free mice were exposed, they developed the hyperactive inflammatory response typical of asthma and IBS. These data parallel the results of another study showing that young children who frequently take antibiotics appear to develop higher rates of asthma and allergies. Although further study is needed, this research demonstrates the importance of what scientists call "proper immune conditioning" early in life and the fact that microbes may play an important role in this process.

What is autoimmunity
Why is autoimmunity more common in women than men
Source: www.sciencedaily.com/releases/2012/03/120322142157.htm