Revisiting the "hygiene hypothesis" in gastrointestinal allergy

Area of Science:

• Immunology
• Allergy Research
• Microbiology

Background:

• The hygiene hypothesis suggests early infections protect against allergic diseases.
• Epidemiologic studies show an inverse relationship between infectious and allergic disease frequency.
• Neonatal immune systems favor Th2 responses, promoting allergic inflammation via cytokines like IL-4, IL-5, and IL-13.

Purpose of the Study:

• To explore the immunological mechanisms underlying the hygiene hypothesis.
• To explain how microbial exposure influences the development of allergic diseases.
• To elucidate the role of Th1 and Th2 immune responses in allergy prevention.

Main Methods:

• Review of existing epidemiologic surveys and immunological research.
• Analysis of immune cell responses (Th1 vs. Th2) to allergens and microbial stimuli.
• Examination of cytokine profiles (e.g., IFN-gamma, IL-2, IL-4, IL-5, IL-13) and their impact on IgE production.

Main Results:

• Infectious agents can induce a Th1 immune response.
• Th1 cytokines (IFN-gamma, IL-2) inhibit IgE production and mast cell degranulation.
• Microbial superantigens can expand Th1 subpopulations, counteracting Th2-mediated allergic responses.
• Th1 responses can convert allergen-specific Th2 cells into allergen-specific Th1 cells.

Conclusions:

• The hygiene hypothesis is supported by the mechanism of Th1 immune response deviation.
• Infectious agents, through their "danger signals," initiate Th1 responses that counteract the Th2 allergic bias.
• This immune modulation by microbial exposure is the fundamental basis of allergy prevention proposed by the hygiene hypothesis.