[Why is the prevalence of allergy increasing? Changed microbial load is probably the cause]

The prevalence of allergic disease is increasing dramatically in industrialised countries. Environmental factors that are putative causes of this increase are to be sought among those that induce a shift in T-helper cell (Th1/Th2) balance toward Th2 immunity dominance, and whose natural occurrence is consistent with epidemiological evidence of regional differences in allergy prevalence. Of such factors, changes in the panorama of bacterial and viral infections, altered intestinal microflora, and changes in dietary habits (particularly increased consumption of omega-6 fatty acids) seem to be the most likely causes of the increased prevalence of allergic diseases. There are also other factors, however, such as diesel exhaust particles, tobacco smoke and environmental toxins, which may partly fulfil the immunological and epidemiological criteria. As several of these putative environmental factors exert powerful effects in vitro, the increase in allergy prevalence is hardly surprising.