Although atopy has a strong genetic component, environtmental factors best explain the recent global trend toward increased prevalence of allergic disease. Predictive factors include the following:
- decreased exposure to infectious disease during early childhood
- changes in diet
- higher levels of allergen exposure
- increased environtmental pollution
Of these factors, variances in exposure to infectious disease appear to have the greatest correlation with atopy. Epidemioligical studies point out a negative association between atopic disease in children and a history of measles or hepatitis A virus infection. It is hypothesized that infections such as these tilt the production of cytokines toward interferon gamma (IFN-γ) and the TH1 cytokines, thereby decreasing production of TH2 allergic cytokines such as IL-4. This theory’s attractiveness may be its ability to explain the global increase in atopy due to decreased infection rates in Westernized regions with aggressive vaccination programs.