The hygiene hypothesis revisited: Does exposure to infectious agents protect us from allergy?

Purpose of review: The increase in incidence and prevalence of allergic disease remains a mystery and cannot be explained solely by genetic factors. The hygiene hypothesis provides the strongest epidemiological explanation for the rise in allergic disease. This review evaluates the recent epidemiological and mechanistic research in the role of infectious agents in the pathogenesis of or protection from allergic disease. Recent findings: Recent literature has extended the epidemiological findings of the protective effect of being born and reared in a farm environment and associates an increased diversity of organisms in house-dust samples with protection from allergic disease. Furthermore, human and animal studies provide increasing evidence for the role of both the innate and adaptive immune systems, including regulatory cells, as mediators of this protective effect. Summary: There is evidence that exposure to some infectious organisms can protect from atopy, whereas other infections appear to promote allergic diseases. The timing of exposure to infection and the properties of the infectious agent, in addition to the genetic susceptibility of the host, play an important role in the future development of allergic disease.