Survival of the fittest: allergology or parasitology?

Allergologists have long recognized that only a small fraction of the environmental and food proteins that we are exposed to give rise to IgE responses. This has raised the intriguing and important question: What makes an allergen an allergen? Many protein allergens have close homologs in metazoan parasites, and as helminth genome information grows, it appears increasingly clear that not only are the immune mechanisms of allergy and anti-helminth immunity closely related, but so are the helminth and allergen proteins that induce them. This suggests that at least part of the secret of what allows some proteins to be allergens may actually lie in the long co-evolutionary relationship between worms and their vertebrate (and latterly human) hosts.