An article in New Scientist discusses the current state of food allergy research.
Perhaps most striking are the regional differences. “Peach and melon allergy is particularly common in the Mediterranean - in Spain and Greece,” says Fern’andez Rivas. Reports from clinics suggest that Iceland is a hotspot for fish allergy and Switzerland has a higher rate of celeriac allergy than elsewhere.
These regional variations are likely to be due in part to differences in eating habits, causing people to be exposed to different allergens. But that alone cannot explain a pronounced north-south divide in the type of apple allergy people experience. In northern Europe, people react to the uncooked flesh of apples, whereas in the south it’s the skin that sets them off, whether it’s cooked or not. 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.