The “hygiene hypothesis," developed since the 1980s, is evolving and gathering evidence as researchers confirm that some parasitic worms appear to help fine tune human immune systems and protect against allergies and autoimmune diseases.
What is the Hygiene Hypothesis?
The scientific proposal known as the “hygiene hypothesis” says that exposure to certain organisms in our natural environment teaches the human immune system which things it needs to respond to and which things it can safely ignore. Allergic reactions and autoimmune diseases such as ulcerative colitis, Crohn’s disease, and multiple sclerosis, are instances where the immune system launches an attack at something that is not harmful, either a substance in the environment or the individual’s own tissues.
The “hygiene hypothesis” results partly from observations that allergies and autoimmune diseases were, and still are, increasing in developed countries while remaining uncommon in parts of the world where sanitation is still lacking. Wherever people tend to come in contact with human and animal waste, and thus with organisms transmitted in feces, they seem to have a lower risk of having an immune system that overreacts.
Are Worms Good for Us?
There’s no question that intestinal and tissue worm infestations cause serious disease--no one is suggesting that we should let them run rampant; however, it seems that the parasites that evolved with us over millions of years do have something to give us in return.
The theory of how worms fine tune our immune systems is evolving as well. Here’s a recent proposed explanation:
- The human immune system comes in contact with foreign organisms continuously—bacteria and fungi living on our skin, in our mouths, in our intestines etc., and common microorganisms in food, water, and the environment—our immune systems have had to learn when to react, and when to ignore the potential threat.
- Until recently, when sanitation became the standard in Western countries, parasites were among the frequently encountered threats. Our bodies had to evolve to tolerate worm infestations because an immune response vicious enough to kill worms would also significantly damage our own tissues.
- Exposure to worms triggers the fine tuning of the immune system and teaches it to tolerate worms. At the same time, the response to tissues typically attacked in autoimmune disease, and to common allergens, is also turned down.
- When the immune system is never exposed to worms and other fecal microorganisms, it doesn’t mature properly and reacts inappropriately, resulting in allergy and disease.
What Diseases Does the Hygiene Hypothesis Explain?
Even if proven to be true, the “hygiene hypothesis” doesn’t explain all cases of allergy and autoimmune disease—there are other things involved as well—and we still don’t know for sure which diseases and which parasites are involved. If the explanation given above is correct, scientists suggest that it may have relevance for Alzheimer’s disease, anxiety/depression, asthma, atherosclerosis, some cancers, Crohn’s disease, eczema, hay fever, multiple sclerosis, Parkinson’s disease, type one diabetes, ulcerative colitis and possibly other diseases (Rook).
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Sources:
"Helminthes and Modulation of Mucosal Inflammation." Elliott, David E., Robert W. Summers and Joel V Weinstock. Current Opinion in Gastroenterology 21 (2005), 51 – 58.
"Parasites and Asthma / Allergy: What is the Relationship?" Weiss, Scott T. Journal of Allergy and Clinical Immunology 105 (Feb 2000), 205 – 210.
"Review Series on Helminths, Immune Modulation and the Hygiene Hypothesis: the Broader Implications of the Hygiene Hypothesis." Rook, Graham A. Immunology 126 (2008), 3-11.
Rosemary Drisdelle - Rosemary Drisdelle has been published many times as a nonfiction writer and several times as a poet. Her first book, Parasites: Tales of ...
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