Allergy Cross-Reactions

If trees make you sneeze, they may also make you react to certain fruits and vegetables.
Allergic Living explores the science behind oral allergy syndrome, nature’s allergic double whammy.

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AS FRIENDS herald the return of spring with the glee of a lottery winner, it’s often hard for the tree allergic to join the celebrating. They know that their foes – birch, elm, maple, alder, poplar and their nasty ilk – have begun to churn out clouds of tiny allergy-causing particles. It won’t be long before the runny nose, the wheezing or the red itchy eyes predictably begin again.

Not only do a third of us battle hay fever, but for a significant proportion of these allergy sufferers, the spring bloom is just the start of their allergy woes. Dr. Antony Ham Pong, an Ottawa allergist and clinical researcher, estimates that up to 10 per cent of the general population has a condition called oral allergy syndrome, or OAS.

It’s a less severe form of food allergy, directly related to pollen reactions, that’s known to set off tingling and unpleasant itching in the mouth, throat and lips. Reactions are caused by a variety of fruits, vegetables, nuts, seeds, and even spices; all of which share allergenic proteins with specific hay fever-causing trees and plants.

“Nobody talks about the foods causing oral allergy syndrome because it’s not considered a life-threatening allergy,” says Ham Pong, the author of several articles educating patients and doctors about OAS. “But it’s actually more common than peanut, milk, egg, and fish allergy.”

In springtime, the big cross-reaction offenders are the birch and alder trees. Depending on where you live, anywhere from 20 to 70 per cent of people who are allergic to birch and alder pollens will also have OAS. Ham Pong estimates about a third of birch-allergic Canadians are affected, but the incidence of OAS is even higher in some European countries. Although OAS is relatively common, he doesn’t think it is increasing, at least not in North America. Rather, doctors have become better at spotting this condition.

The Common Denominator

But how can a tall, skinny tree that gives you the sniffles cause an itchy mouth if you chew on a celery stick? Unfortunately for those with tree allergies, the pollen proteins that trigger hay fever symptoms are also found in a grocery list of foods. The components of those proteins are well conserved among different plants, which means their three-dimensional structures are so similar that some of our immune systems can’t tell the difference between, say, birch pollen and apple peel.

In birch, the most common allergy troublemaker is a protein called Bet v 1, and alder has a similar protein called Aln g 1. Their purpose is to help protect the fruit or vegetable in times of stress or from infection. Ham Pong explains that Bet v 1 is called a “pan-allergen” because very similar proteins are found in a variety of unrelated plants, including apples, plums, kiwis, carrots, celery, potatoes, hazelnuts, almonds, sunflower seeds, and even spices such as oregano, basil and dill.

People with birch-related OAS already produce antibodies to the pan-allergens from their prior exposure to birch pollen, so those antibodies will try to protect against similar proteins when they enter the mouth.

Pan-allergen relationships don’t stop with birch trees. The allergenic components of grass pollen are also shared with melons, oranges, kiwi, tomatoes and peanuts, among others.

Those who suffer in the fall from ragweed allergies (mainly in the northeast of the continent) can get itchy mouths from banana, melons, zucchini and cucumber. For those reactive to ragweed, serious cross-reactions are also possible from consuming chamomile, honey and echinacea. This is not OAS, however. In this case, it’s because they belong to the same botanical family.

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