REVIEW ARTICLE Does allergy impair innate immunity? Leads and lessons from atopic dermatitis

The original ‘hygiene hypothesis ’ was built on epidemiologi-cal data that reduced early childhood infections because of improved living and hygiene standards can increase the risk of acquiring allergic diseases (1). Immunological attempts in support of this hypothesis have argued that high levels of endotoxin and extracellular polysaccharides can lead to pro-tective Th1 immunity and, subsequently, to reduced rates of Th2-driven allergic conditions (2, 3). For example, Perkin and Strachman (4) showed that unpasteurized milk consump-tion was associated with significantly lower IgE levels and fewer allergy symptoms. Other studies have suggested that ingestion of probiotic bacteria (Lactobacillus, Bifidobacteri-um) or fermented milk products or exposure to environmen-tal mycobacteria may be helpful in preventing allergies (5–7). Interestingly, many measures which supposedly protect fro