TH2 cell-mediated immune responses against "innocuous" antigens play a triggering role in atopic allergy. Several epidemiologic studies have clearly shown that the reduced microbial exposure of children caused by the westernized lifestyle is responsible for the increased prevalence of allergy that has occurred in the last decades In developed countries ("hygiene hypothesis"). The immunologic changes caused by the reduced exposure to pathogenic and nonpathogenic microbes during childhood are still controversial. The initial interpretation has been a lack of shift of allergen-specific responses from the TH2 to the TH1 phenotype. This is because of reduced production of IL-12 and IFNs by cells of the natural immunity stimulated by bacterial products through their Toll-like receptors (missing immune deviation). Another interpretation emphasizes the importance of reduced activity of T-regulatory cells (reduced immune suppression). However, although there are impressive amounts of data in favor of the missing immune deviation, experimental evidence supporting the role of reduced immune suppression in explaining the increased prevalence of allergy is currently weak or even contradictory. The solution to this question is very important not only from a theoretic point of view but also because of its therapeutic implications.
Romagnani, S. (2004). Immunologic influences on allergy and the TH1/TH2 balance. Journal of Allergy and Clinical Immunology, 113(3), 395–400. https://doi.org/10.1016/j.jaci.2003.11.025