A Symbiotic Tale: The Hygiene Hypothesis as It Stands Today
The story of our symbiotic relationship with microorganisms begins with the advent of human life on earth, when man lived as a hunter-gatherer. In the wild, the lifestyle of early humans echoed with the throbbing pulse of nature; their means of cooking and storing food were primitive.
As a result of close interaction with their environment, humans developed an extensive gut flora: the horde of microorganisms present in food colonized the human gut. The harmful and disease-causing bacteria among these were what we call “bad flora”, while “good flora” entered a mutually beneficial relationship with its human host. (1)
It is central to the following discussion to understand that our bodies are ecosystems that have evolved over millennia. One way in which our gut bacteria influence our biology is by promoting development of our immune system.
Exposure to infections in early childhood “schools” immune cells to recognize foreign microbes and to make antibodies against them, while the body’s own cells are recognized as ‘self’ and therefore escape the same immune defenses. The latter phenomenon is known as establishment of immune tolerance to self antigens.
The ‘Hygiene Hypothesis’ – also called Biome Depletion Theory and Lost Friends Theory – elegantly captures this idea. First put forward by Strachnan in 1989, it states that a lack of early childhood infections increases susceptibility to allergic diseases by suppressing the natural development of the immune system.
In recent decades, with industrialization and globalization leading us to campaign for a ‘cleaner and greener’ world, standards of hygiene have improved dramatically world over. While countering the infectious disease burden, this has been accompanied by soaring rates of allergic and autoimmune diseases.
In simple terms, the explanation offered is this: when the body is not exposed to pathogens that it is trained to combat, immune cells instead overreact to harmless stimuli (in case of allergies), and begin to attack the body’s own cells (in autoimmune and chronic inflammatory disorders). (2)
Over the years, this hypothesis has engendered research and yielded more insight. We now realize that not all infections are helpful in immune development. Rook stated, in 2003, that it is not childhood infections like measles or chickenpox – which appeared fairly recently in human history – that are our ‘old friends’; rather it is the microbiota that has resided in the gut since more ancient times, and certain helminthic parasites, that aid in immunoregulation. (3)
While this by no means suggests that we should grow lax about maintaining proper hygiene protocols, these findings provide scientists with a direction in which to focus research efforts. Helminth therapy – involving deliberate inoculation with modified intestinal parasites – has emerged as an experimental type of immunotherapy.
In 2007, animal studies showed that helminth therapy protected against inflammatory conditions such as Graves’ hyperthyroidism, multiple sclerosis, Type I diabetes and asthma. (4)
Although Elliott et al have drawn encouraging conclusions, a study conducted in Denmark by Bager et al in 2012 gave conflicting results. (5) Scientists have yet to determine the clinical dose and specific species of worms most likely to be safe and effective. (1)
Apart from this, probiotics are used to introduce helpful bacteria into the body but their efficacy is still questionable. Though at present we are far from completely understanding and harnessing the potential of traditional pathogens, we hope the Hygiene Hypothesis can take us far in this quest.
1. Vogelzang JL. The History of the Hygiene Hypothesis. 2013. http://works.bepress.com/jodyvogelzang/4/
2. Wikipedia. Hygiene Hypothesis. Mar 2016. https://en.wikipedia.org/wiki/Hygiene_hypothesis
3. Rook GA, Martinelli R, Brunet LR. Innate immune responses to mycobacteria and the downregulation of atopic responses” Curr Opin Allergy Clin Immunol 2003 Oct;3(5) 337-42.
4. Elliott DE, Summers RW, Weinstock JV. Helminths as governors of immune-mediated inflammation. International Journal for Parasitology. 2007; 37:457-464.
5. Bager P, Hansen AV, Wohlfahrt J, Melbye M. Helminth infection does not reduce risk for chronic inflammation disease in a population-based cohort study. Gastroenterology. 2012; 142:55-62.
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