At the Vienna meeting, researchers discussed the patterns emerging from their research. For adults and children over 3 years old, hazelnuts and apples turn out to be the most common triggers of food allergies in Europeans reporting to clinics — not peanut allergy, as many people might expect.
These reports from clinics have also thrown up a surprising new player: sunflower-seed allergy. Although something of a rarity, it may become more common as sunflower seeds are increasingly appearing in food. To make matters worse, the allergen involved seems to be particularly potent. “The proportion of severe reactions is higher than for peanut,” says Montserrat Fernández Rivas, an allergologist from the San Carlos Clinical Hospital in Madrid, Spain.
Perhaps most striking are the regional differences. “Peach and melon allergy is particularly common in the Mediterranean — in Spain and Greece,” says Fernández 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. 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.
A similar cross-reaction explains the allergy to apple skin found in southern Europe. In this case, a prior sensitisation to the Pru p 3 protein in peaches, which bears a strong similarity to Mal d 3, seems to be the culprit. What’s more, Mal d 1 breaks down when heated while Mal d 3 is heat resistant, which neatly explains why northern Europeans are fine with cooked apples and pasteurised apple juice but apple-allergic people in the south cannot cope with these fruit in any form.
Other reported cross-reactions include a link between house-dust-mite faeces and shrimp allergy, and another between mugwort pollen and an allergy to carrots, celery and sunflower seeds. There are likely to be many others, since many allergens seem to share similarities in their amino acid sequences that might confuse the immune system.
In fact, between 2005 and 2008 Mills and Heimo Breiteneder, a molecular allergist at the Medical University of Vienna, and their colleagues completed a series of studies showing the majority of allergens originating in fruit and vegetables belong to just four of the 10,000 or so recognised families of proteins, and most of the animal-food allergens to just three families (The Journal of Allergy and Clinical Immunology, vol 115, p 163 and vol 121, p 847). Bet v 1, for example, causes cross-reactions with several other members of its protein family, and as a result people who have birch pollen allergy stand a good chance of being allergic to apple, celery, plums and several other common foods.
This also explains why some migrants from east Asia to northern Europe suddenly develop an allergic reaction to jackfruit once they have come into contact with birch pollen. The allergen in jackfruit does not on its own sensitize the immune system, but once birch pollen has done the job, the immune system may react to jackfruit too.
These numerous examples of cross-reactions raise another question: why does Bet v 1 cause an allergy to the Mal d 1 protein but not the other way around? Researchers believe it’s because Bet v 1 enters the body via the lungs, so it is not broken down by digestion and can reach the bloodstream intact, where it activates the immune system. Mal d 1, on the other hand, is broken down during digestion, so it loses its capacity to prime the immune system. Once the immune system has been stimulated by the Bet v 1, it may then become sensitive to similar looking proteins like Mal d 1 — sensitive enough to trigger a reaction when it comes into contact with the mouth.