As anyone who has any dealing with allergy will know, accurately estimating prevalence is largely a guessing game. Much of the data comes from notoriously unreliable self- reported studies; even when medical testing has taken place, methods, and therefore results, are rarely uniform.
As a result, a meta-analysis in 2007 by the Technical University of Denmark and King’s College London suggested that medically tested allergy prevalence varied from one person in 100 to one in 20; in self-reported surveys the variation was from one in 30 to more than one in three.
It was to investigate these anomalies and the regional differences between allergies that the EuroPrevall study was set up in 2005 involving over 60 partners not only across Europe but also in Ghana, Australia, India and China. At a recent meeting of the group in Vienna emerging allergy patterns were discussed – some of them quite surprising.
For example, for anyone over three years old, hazelnuts and apples appear to be the most common allergy triggers while the most serious threat is not peanuts but sunflower seeds, reactions to which are frequently more severe than to peanuts. Meanwhile, peach and melon are the most common allergens around the Mediterranean, the Swiss are particularly sensitive to celeriac – and Icelanders to fish!
The north/south divide
However the most interesting finding so far is the strange north-south divide which runs across southern France, through Italy just north of Florence and on eastwards to the Black Sea. North of this line people react to the flesh of apples, south of the line, to the skin. The answer to this conundrum may lie, yet again, with the birch tree as this line also marks the latitude south of which the birch does not grow.
The birch connection – again...
Apples contain two major allergenic proteins, Mal d 1, which is found in the flesh, and Mal d 3, which is found in the skin. Mal d 1 closely resembles the allergenic protein Bet v 1, found in birch pollen. So, anyone who is already allergic to Bet v 1, is ‘primed’ to react to Mal d 1 – explaining why people living in birch growing areas and sensitised to birch pollen may also react to apple flesh.
South of the birch growing area, the apple protein Mal d 3 is closely related to Pru p 3, the main allergenic protein in peaches. And, as the EuroPrevall study has already noted, the prevalence of peach allergy is highest in the Mediterranean area.
Interestingly, Mal d 1 breaks down when heated, where as Mal d 3 does not – which could explain why apple-allergic northern Europeans are fine with cooked apple or pasteurised apple juice, but southern Europeans cannot tolerate apple in any form.
The question of why those who are birch sensitised should react to apple flesh, but those who are apple sensitive do not necessarily react to birch pollen has also baffled researchers. They believe that this may be because the Bet v 1 from birch pollen enters the body via the lungs rather than the digestion. It can therefore reach the bloodstream intact. Had it been eaten, it would have been broken down by stomach acids etc and would have lost the ability to sensitise the immune system. But once the immune system is sensitised by Bet v 1 it is likely to react to any similar looking proteins such as Mal d 1.
Other cross-reactions have also been reported – between house dust mite faeces and shrimp, and mugwort and carrots, celery and sunflower seeds and many more are expected to emerge. Birch protein is also related not only to apple but to celery, plums, jackfruit and a number of other common foods. Surprisingly the majority of allergens in fruits and vegetables belong to just four of the thousands of families of proteins, and most animal food allergens belong to just three families.
Drawn from an article in the New Scientist, 1st August 2009
First published in 2009
• If this article was of interest you will find many other articles on unlikely allergies and allergy connections here – and links to many relevant research studies here.
• For more on the more 'mainstream' allergies check in to our 'allergy and intolerance home page' – and for ideas on alternative foods go here.
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