Infections in early life and susceptibility to allergic diseases: relevance of hygiene hypothesis

Allergic manifestations such as rhinitis, asthma and eczema are increasing all over the world and more steeply in western industrialized countries during the last few decades. The underlying causes are poorly understood. Genetic factors are unlikely to explain the increased prevalence of allergic diseases as genetic shift in the population cannot occur in such a short duration. Changes in lifestyle, living conditions, food habits and environmental factors are implicated in the increased prevalence of allergic diseases. One of the explanations deduced from hygiene hypothesis is that early-life infections may protect against allergic sensitization. Changes in lifestyle have led to decreased exposure of children to certain infections, which is responsible for the increase in prevalence of allergy and asthma. There is growing evidence that under certain situations exposure to microbial products may instead protect against inflammation. This was further supported by cross-sectional studies involving farming and non-farming households in which concentration of endotoxins exposure showed inverse association with allergic manifestations. This article examines the relevance of hygiene hypothesis on the basis of current knowledge of molecular mechanism of allergy and reviews the recent literature on the role of infections in early-life, in maintaining immune homeostasis.