Food Allergy? It May Be in the Air You Breath

As astounding number of different foods cause allergic reactions in at least some people, foods that seem to have little in common. Not only that but food allergies are often strangely specific and regional in nature. The deadliest allergies are not necessarily the commonest allergies and the percentage of people who react to allergens also varies. People often don't know if they have an actual allergy, but their suspicions run rampant: self-reported allergic reactions may be 30 times higher than reactions found by testing.

Time to bring in the science. Probably the biggest group looking at the mysteries of allergies is EuroPrevall:

EuroPrevall is an EU-funded multidisciplinary integrated project (IP) involving 17 European member-states, Switzerland, Iceland, and Ghana. Of the 63 partners, there are 15 clinical organisations and six small-medium sized enterprises (SMEs) as well as the leading allergy research organisations in Europe. Since the project began new partners have also joined from New Zealand, Australia, Russia, India and China.

The website isn't a thing a beauty. I've run in circles trying to find a supposedly downloadable brochure, with no luck.

With luck, the researchers can find their way around the scientific side more easily. One major approach EuroPrevall has taken is to study the regional differences in allergies, hoping to find clues to the way they occur. Andrew Watson wrote about one major and fascinating result in this week's New Scientist magazine, Food allergies get curiouser and curiouser.

Southern Europeans, in countries bordering on the Mediterranean, react to apple peels, cooked or uncooked, including the remains in processed apple juice. Those north of a sharp line running across the bottom of France, mid Italy, and northern Greece only react to the uncooked flesh. After age three, apple allergy is one of the commonest complaints of those visiting clinics.

What sense could this possibly make? That's where the comparative studies prove their worth. That sharp line marks the southernmost limit of areas where birch trees grow. Birth tree pollen is a common cause of hay fever. That line is also the northern boundary of a line of peach allergies.

When the scientists look at the individual proteins they find two allergens in apples: Mal d 1 in the flesh and Mal d 3 in the skin.

Here's the connection. The protein Mal d 1 is similar to the birch pollen allergen Bet v 1. The peach protein Pru 3 p is similar to Mal d 3. People get sensitized by one kind of pollen, priming the immune system. When they eat the similar protein in another food the immune system reacts.

The direction of the connection can also be explained. People breathe in birch pollen, sending the pollen directly into the bloodstream. This bypasses the digestive tract so it doesn't get broken down and denatured. But a later bite of the apple triggers a reaction as soon as the protein makes contact with the mouth.

In more formal terms:

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...

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.

The EuroPrevall teams are finding additional examples of this cross-sensitization:

a link between house-dust-mite faeces and shrimp allergy, and another between mugwort pollen and an allergy to carrots, celery and sunflower seeds ... 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.

With pre-primed individuals suddenly coming into contact with foods they might not other have eaten - sunflower seeds, like soybeans, are used extensively in the food industry - allergies may start erupting where never before seen.

As always, knowing a possible underlying feature of a problem gives hints at a solution but doesn't provide any ready answers. Taking some of the mysteries out of allergies still is a welcome first step.