Correspondence Address:
Babak Beikzadeh
Department of Cell and Molecular Biology and Microbiology, Faculty of Biological Sciences and Technology, University of Isfahan, P. O. Box: 8174673441, Azadi square, Isfahan
Iran

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To the Editor,

After 30 years, the Hygiene Hypothesis is still being debated in the scientific community. The essence of Hygiene Hypothesis is that exposure of the immune system to microbes in early life prevents allergic disease later in life.^[1] In other words, people who live in rural areas have a low risk for allergic disease than in urban areas. The concept of this hypothesis backs to the balance of the Th1/2 response.^[1] According to Nitya Jain's review, the early life microbiota is formed during the first two years of life and educates the immune system to be involved in health and disease for the rest of its life.^[2] But there's a question: does early life microbiota and Th1 polarization could dominant until the end of life? If we look at children in rural areas, in addition to microbial antigens, they also encounter allergens, such as mites, pollens, and intestinal helminth infections, which indicates the polarization of responses to Th2 response.^[3] The effect of helminths on allergic disease varies depending on the species, infection duration, and parasite load. Some of them reduce the risk of this disease (Schistosoma mansoni), whereas others induce or develop allergic symptoms such as Ascaris lumbricoides.^[4] On the other hand, the presence of a high level of pathogenic bacteria (Staphylococcus aureus and Clostridium difficile) and also Anelloviruses have a positive correlate with the allergic disease, while good bacteria such as Bifidobacterium lactis Bb-12 and some Lactobacillus spp. such as Lactobacillus GG develop anti-inflammatory responses in the gut.^{[1],[2],[5],[6],[7]} Moreover, immigrants who have migrated from low allergy prevalence areas to high allergy prevalence countries acquire an allergy sensitivity profile over time.^[8] Furthermore, probiotics can help to repair the microbiota, balance the Th1/Th2 response, and reduce allergy symptoms.^[5] Cumulative evidence suggests that only attention to the microbiota in early life is not enough for the rest of life. As a result, if we want to have a normal immune response that reduces the risk of allergies, we must guarantee the presence of a good microbiota throughout life.

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