Monday, April 16, 2012

The Hygiene Hypothesis

There are currently many products on the market that claim to be effective household antibacterial agents that are purported to have health benefits by preventing exposure to infectious agents.  Although this may have value to some extent, these claims may, in fact, be misleading in relation to overall public health.

There is a growing body of scientific evidence that strongly suggests that normal human exposure to the microorganisms that pervade the natural environment may be of value to human health by helping to fine tune the human immune system and thereby prevent the unset of the types of immunological overreactions that are typified by asthma and some autoimmune diseases (an autoimmune disease is a condition that is a result of the human immune system attacking its own tissues).  This concept is referred to as the Hygiene Hypothesis.  According to this hypothesis, it is vital that the human immune system be normally exposed at a young age to the microbes that are so ubiquitous in the environment.  Without these kinds of interactions, it is posited that later in life the immune system would be prone to behave in such a way as to result in inflammatory and autoimmune reactions such as allergies, asthma, inflammatory bowel disease (IBS) and multiple sclerosis (MS).  Epidemiological studies have clearly shown that children growing up on farms are considerably less likely to suffer from allergies and asthma as compared to their urban counterparts.  This correlation, however, does not constitute proof given that there are many other variables involved.

The Hygiene Hypothesis has been given further credence as a result of the findings of Dr. Richard Blumberg, an immunologist, from the Brigham and Women's Hospital in Boston.  Blumberg and his colleagues studied mice raised in germ free cages and fed germ free food.  These mice are more susceptible to asthma and colitis - an inflammatory condition of the gut.  On exhaustive examination, they discovered that these animals had elevated levels of a rare immunological cell type - so-called invariant natural killer T cells (iNKT) in the lungs and the intestines.  These cells have been shown to trigger inflammatory responses following exposure to microbes or antigens – molecules that can trigger the immune response – produced by the body.  It should be noted here that the mouse animal model has been proven to be a good one; since, there is a very close correspondence between mouse and human immunology.  In addition, mice that had been genetically altered to lack iNKT cells do not exhibit colitis even when raised in a germ free environment.  Furthermore, when germ-free mice are transferred to a normal environment at an early age, they show a normal distribution of iNKT cells.

These are very interesting findings that show a mechanistic rationale for the Hygiene Hypothesis.  Hopefully, this kind of information will help individuals make more informed decisions regarding the use of anti-bacterial agents in the home. 

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