The Hygiene Hypothesis

"Wake yourself up! Don’t erase me.”
Clementine Kruczynski, Eternal Sunshine of the Spotless Mind
In a modernized society, all of our upgrades and technology and cleanliness can seem like blessings without any drawbacks. However, a few interesting trends have developed in the last couple decades concerning our health. While we've been able to lower the incidence of diseases like Measles and Mumps, other autoimmune and allergy related diseases are on the rise. Oddly enough, well-developed and industrialized countries show increased rates of Type 1 Diabetes (formerly known as juvenile diabetes, an autoimmune disease) incidence compared to less-developed countries. Children raised in farms and near barns have a lower incidence of asthma as compared to city children1.
  Type 1 Diabetes development is following an interesting pattern2.  
  New diseases are on the rise3.  
So what’s going on? Why are these types of diseases suddenly increasing? In recent decades, the hygiene hypothesis has been gaining popularity. This theory was developed because David Strachan observed, in 1989, that hay fever and eczema (allergic diseases) were less common in larger families, potentially because there was more exposure to infectious agents. As some countries become cleaner and families move away from rural areas to urban locations with better sanitation, there can also be less exposure to these infectious agents.
Another theory, termed the Old Friends Hypothesis (coined by Graham Rook) states that the contributing factors in these scenarios are the microbes we’ve co-evolved with for tens of thousands of years. These old friends have become symbiotic with us and help prime us for appropriate immune responses. Infectious diseases are unlikely to be as helpful, as they either kill the host or are immunized against, and therefore, cannot become a part of our "circle of friends". Essentially, we cannot function optimally without interaction with our old friends.

Arrested Development®, Netflix.
The exact immunological explanation of how early exposure to these microbes tolerizes us or improves our health has not been completely defined yet. Early exposure to microbial antigens might educate Tregs (immunosuppressive cells you can learn about here) and help to dampen inappropriate immune responses. Others suggest that a lack of infections prevents long lived memory T cells from developing. Then, when the body finally has need of more T cells (i.e., after an infection wipes out some of your T cell population), homeostatic proliferation may be biased toward allowing auto-reactive T cells to multiply, leading to autoimmune disease4.

A little dirt may do you some good.
Whatever the mechanism, it’s clear that we have to consider environmental factors in addition to genetic ones when it comes to studying diseases. For instance, antibiotics can wipe out or drastically alter your gut microbiome. The diet itself can influence what types of bacteria occupy niches in the gut. How will this affect your overall immune response? Do we stop obsessively using hand cleaning products? We may soon find that our microbial friends may have a reach far beyond our digestive tract when it comes to health. Do you have any thoughts on the hygiene hypothesis? Contact us here.
  1. Asthma and Farms
  2. Type 1 Diabetes Incidence
  3. Strachen’s Hygiene Hypothesis
  4. Homeostatic Proliferation
  Just when I thought I got the hang of science...  
Contributed by Ken Lau, PhD.
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