Investigation of the hygiene hypothesis: current issues and future directions

Abstract

The hygiene hypothesis was first proposed in 1989 by Strachan et al. (1) who proposed that reduced opportunities for cross-infection in families may have resulted in the more widespread clinical expression of atopic disease. It was based on the observation of a striking inverse association between sibling number and hay fever in adulthood among those in the 1960 UK birth cohort study. It proposed that sibling number could be protective for allergic disease because siblings are the source of infection. The immunological mechanisms that might mediate the consequences of increased sibling exposure are unclear and the simplistic proposal that early life microbial deflects the immune system from a Th2 (allergic)- to a Th1(nonallergic)- response can be challenged.