What Can Be Learned from Observational Epidemiology?
It seems beyond doubt that IgE sensitization to an allergen requires exposure to that particular allergen. Not surprisingly, the pattern of sensitization in a particular population reflects the allergens prevailing in the environment. Hence, the prevalence of sensitization to olive pollen is high in Mediterranean regions (e.g. Southern France and Italy), but low in Scandinavia (e.g. Sweden and Norway), while the prevalence of sensitization to birch tree pollen is low in Mediterranean regions, but high in Scandinavia [45]. Nevertheless, at the individual level within each community there is often no clear relationship between exposure to allergens and risk of sensitization. Many epidemiological studies have looked at the relationship of pet keeping to IgE sensitization against pets and allergic disease. It has generally been assumed that pets confer an increased risk of sensitization and atopic disease. However, during the 1990s a number of studies emerged reporting a negative associati
