There is no doubt among experts that the prevalence of allergic diseases has increased in many industrialized countries in recent years. The rea sons for this increase are unknown; only suppositions exist. Many people focus on environmental influences. However, the assumption that air pollution alone is responsible for this increase seems to be too simple: many other influences, including the genetic predisposition of individual patients, allergen exposure, and possibly socioeconomic factors, also have to be taken into consideration. Although our understanding of the complex mechanisms of...