Environmental Exposure to Endotoxin and Decreased Risk of Childhood Atopy

The hygiene hypothesis implies, in immunological terms, that microbial stimulation in early life skews the immune system towards the development of a T helper type 1 (Th1)-type lymphocyte population, and away from a T helper type 2 (Th2)-type development that is associated with atopy. The hygiene hypothesis originally theorized that increased infections protected against atopy. This theory has evolved since harmful infections may not be as critical as exposure to microbial burden because exposure to microbes can occur in the absence of infections. Despite substantial public hygiene measures in modern metropolitan communities, an atopy-protective effect of endotoxin in these metropolitan environments still occurs. This article is a review of clinical and experimental studies concerning the protective effect of endotoxin exposure on the susceptibility to atopic responses. The discussion includes : (1) factors influencing household endotoxin levels ; (2) recent developments in the potential use of endotoxin for the prevention of atopy and asthma ; (3) association of endotoxin and pets with atopic sensitization and diseases ; (4) animal studies on the protective effects of timing of endotoxin exposure on atopy and asthma and (5) animal studies on the protective effects of endotoxin dose on atopy and asthma.