Recent studies show that food allergies in Europe have a regional differences. An early hypothesis was that the different diets from the varying regions explained the variety of allergies. It did not explain certain foods, like apples. In northern Europe people are allergic to the inside of the apple, and in the south they are allergic to the skin.
What could be the cause of this strange invisible dividing line that skims across south-west France, cuts through Italy close to Florence, and continues eastwards through the middle of the Black Sea?
Significantly, this line marks the southern limit of the birch tree, a plant whose pollen is one of the causes of hay fever in northern Europe. Clues for this link lie in the different proteins found in various parts of the fruit: the flesh harbours an allergenic protein called Mal d 1, while the skin is relatively rich in Mal d 3. The structure and composition of the Mal d 1 protein strongly resembles the allergenic protein Bet v 1 found in birch pollen. This means that people who suffer from birch pollen allergy may be primed to overreact to Mal d 1 – explaining the prevalence of the allergy to apple flesh in this region.
Inhaling a protein may cause someone to develop allergies for a similar ingested protein, but not vice versa because when it hits the lungs it passes into the bloodstream whole, and is not broken down by digestion.
There are several factors at work here, including genetics and other environmental conditions which have not yet been analyzed. Hopefully, with more research we will soon see the end of severe allergies.