A higher exposure to endotoxin was hypothesized to contribute to protective effects on atopy development by activating type-1 lower prevalence of allergic sensitization and hay fever in children T-helper cell responses on the other (9). growing up on a farm. We studied the association between house Endotoxins are more or less ubiquitous in the environment dust endotoxin and allergic sensitization. We randomly selected and are present in normal indoor environments as constit-740 children, aged

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... tween 5 and 10 years, from a group of children uents of house dust (7, 10-12). Increased concentrations of who participated in two cross-sectional surveys performed in Saxhouse dust endotoxin were found in homes of farming famiony-Anhalt, Germany, from 1992 to 1993 and from 1995 to 1996, lies, in households where children had regular contact with such that 50% of the children were atopic or had a diagnosis of farm animals (13), and where animals were kept indoors (5, asthma. From 1996 to 1998, we collected living-room floor dust in 11, 12, 14). the homes of 454 of these children (61%). The content of endotoxin Several cross-sectional studies have shown a consistently in house dust was quantified using a chromogenic kinetic limulus lower risk of allergic sensitization (15-20), allergic rhinitis, amoebocyte lysate test and was related with health outcomes meaand hay fever (15-17, 21, 22) in children who grew up on sured in the preceding cross-sectional surveys. Multiple logistic farms. Other studies have shown a protective effect of exporegression analyses adjusted for place of residence, sex, age, parensure to pets on cat allergy, in particular, for children with tal education, parental atopy, and pet ownership showed a negaatopic heredity (23-25). Nafstad and coworkers (26) reported tive association between exposure to endotoxin and sensitization protective effects of exposure to pets early in life on the to one or more allergens (aOR [95% CI] 0.95 [0.83; 1.10]) and two development of allergic diseases such as atopic eczema and or more allergens (aOR [95% CI] 0.80 [0.67; 0.97]) using 0.35 kU/L allergic rhinitis (26). Furthermore, one cross-sectional study as the cutoff value for sensitization. The protective effect was has demonstrated a negative association between presence strengthened with increasing degree of sensitization. In conclusion, of pets in the home during childhood and adult atopy (27). exposure to higher levels of house dust endotoxin is associated Therefore, it was hypothesized that the level of environmenwith lower prevalence of allergic sensitization in children. tal exposure to endotoxin and other bacterial wall components early in life might be protective in the development of