The hygiene hypothesis suggests that smaller family sizes as well as the use of antibiotics and vaccines has led to an allergy epidemic. Probiotics could reverse that epidemic and reduce the risk of developing atopic diseases and, potentially, nasal and ocular symptoms of respiratory allergies.
That concept has since become known as the "hygiene hypothesis" and is widely used to explain the allergy epidemic. If our cleanliness is, indeed, the bane of our immune systems, then might it be possible to re-direct the immune response through exposure to bacteria, allergens, and similarly unpleasant things?
Allergy is undoubtedly more common in children who live in the city than in their farm-dwelling counterparts.3 Early exposure to pollen, as well as animal-associated dander and feces, is believed to encourage the maturation of the immune system. This "allergy immunity," however, does not always survive if the individual relocates to the city,4 a possible effect of pollution intensifying the potency of pollens.5
Allergy also appears to be less common in individuals infected with Schistosoma mansoni, a species of helminth parasite that secretes anti-inflammatory cytokines.6 Th2 responses, specifically eosinophil recruitment,7 likely evolved as protective responses to parasitic worms. Evolution is a constant battle for the winning spot-and the helminthes' responses to Th2 reactions are anti-inflammatory cytokines. Because most people aren't willing to trade their allergies for intestinal parasites, some researchers have been attempting to purify the helminth-produced anti-inflammatory chemokine.6
Understandably, a more palatable alternative is desirable. Microbial colonization of the intestine begins shortly after birth, and our symbiosis with the hundreds of species of gut-residing bacteria is vital to the normal functioning of our digestive and immune systems.8 Through experiments with germ-free mice, researchers determined that the establishment of intestinal flora as a newborn is crucial to the development of a Th1-Th2 system that can be regulated correctly.9 When antibiotics are administered to infant mice, the gut flora is altered, and the Th1 immune responses are impaired.10