Too clean for our own good?

When my daughter was 2 months old, we took her to a work party. She was passed around and adored. Meanwhile I was cringing behind a waffling smile thinking, “None of those people washed their hands!…I bet they are all sick!..Did that person just sneeze?!”.  The minute she was back in my arms I gave her a thorough wet-wipe wipe down.  My husband looked at me like I was crazy.  I am not a germaphobe; I just wanted to protect my infant child from all of those nasty germs, even though I am big fan of the hygiene hypothesis.  This theory has been around for decades and essentially states that early exposure to microbes and allergens at a young age helps the immune system develop so it doesn’t over-respond later and prevents the develop of things like asthma later in life.

Certain trends support this hypothesis.   Having more children (3+), which is becoming more uncommon, is believed to expose the younger children to more germs.  These kids tend to develop less atopic conditions.  Consistent with this, children in daycare tend to develop fewer allergies than those who are never in such environments.  Children who are breastfed or that grow up on farms typically have fewer incidences of asthma and allergies.  In addition, the increased use of antibiotics leaves us less exposed to microbes in general.  It seems as though the Westernized, clean, environment we are providing for our children tends to be too clean, and is not providing proper immunological education.

There is definitely a debate about the hygiene hypothesis, and the research behind it is not complete.  However, a recent study by researchers at Harvard Medical School have identified a role for early microbial exposure and colonization with conventional microbiota in preventing inflammatory bowel disease (IBD) and mucosal inflammation.   They utilized a germ-free mouse model (mice born and housed in sterile conditions without any pathogens or microbial colonization) and introduced microbes at various ages followed by induction of IBD or asthma. The authors found that germ-free mice had greater colitis and asthma than mice that had been colonized (but pathogen-free) and that these effects were mediated in part by an influx of immune cells called invariant natural killer T (iNKT).  iNKT cells express a molecule called CD1d that can recognize different components either on microbes or on self-host cells, and have the capacity to respond quickly and vigorously. So in the case of mucosal inflammation, as with any immune response, we would like them to be controlled. This signal that germ introduction induces for a normal immune response needed to happen in the first 2 weeks of life (for mice).  Germ-free mice colonized as adults were not protected from the increased severity of IBD, suggesting that early exposure to microbes is necessary for normal immune development.

Don’t get me wrong, I am still a huge advocate for frequent hand-washing, but the more research I read in support of the hygiene hypothesis, the less I cringe when I see my daughter eat those stale cheerios off the floor.

Reference: Microbial Exposure During Early Life Has Persistent Effects on Natural Killer T Cell Function. Olszak T et al.  Science. Published online 22 March 2012


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