The dramatic increases in allergy and allergic diseases which have occurred over the past half-century have
been strongly associated with changes in early life events. Changes in lifestyle, which have affected nutrition,
allergen and microbial exposures, have been strongly implicated as explanations for changing allergy
susceptibility. To what extent these influences affect the foetus during pregnancy rather than the infant post-
natally is uncertain. In all probability there is a sequence of events through pregnancy and early post-natal
life which in combination change the outcomes. Successful pregnancy is associated with a Th-2, allergy-
biased immunological environment which results in virtually all neonates having an apparent allergic pattern
of immune responsiveness. Under normal circumstances this is rapidly down-regulated after birth, most likely
affected by the evolving human microbiome. Interactions between genetic and environmental variation, both
during pregnancy and in the first months of the infant’s life, perturb this normal sequence and lead to a
sustained Th-2-biased allergic response.
The future for prevention rests in identifying the key influences that should highlight strategies to turn the clock
back to an era of low prevalence of allergic diseases.