Why do certain fruits and vegetables cause allergic reactions?
The pollen-food syndrome (or oral allergy syndrome) is an allergic reaction to fresh fruits, vegetables and/or nuts that can happen in people that are allergic to pollen. Most symptoms occur in or around the buccal cavity, which is why the syndrome is called as such. This entity can also occur in children. The pollen-allergy syndrome is a type I hypersensitivity and is mediated by antibodies, more specifically IgE.
Certain fruits, vegetables and nuts share similar proteins with the ones found in pollen. When the antibodies specific to a particular pollen cross react with certain food ingested by an individual, it triggers an inflammatory reaction that will lead to the allergic reaction. The most common association of the pollen-food syndrome is with birch tree. The fruits and vegetables most commonly related to birch tree are apples, peaches, prunes, pears, cherries, apricots, carrots, celery, almonds, hazelnuts and peanuts. (see also the website Canadian Food Inspection Agency for a list). Of interest in this syndrome is that the proteins responsible for this particular reaction are sensitive to heat and to digestive enzymes. This therefore explains why most people affected by this are able to tolerate the cooked form. However, this does not apply to nuts.
When we are faced with patients that are not allergic to pollen, the proteins that are responsible for these allergies are different. They are much more stable to changes in temperature and also resist gastric enzymes and therefore can cause more severe reactions..
Can the symptoms lead to anaphylaxis?
The pollen-food syndrome is so-named because the majority of the time the symptoms are limited to the mouth and throat. Again, because the proteins are sensitive to gastric enzymes, they are rapidly degraded upon ingestion, which limits the extent of the reaction. Usually, the reaction occurs within minutes of eating. Patients usually complain of itchiness and sometimes swelling. It is also possible to notice an increase in the symptoms during the pollen season. In the literature, it is noted that the risk of systemic reaction is less than 10% and the risk of an anaphylactic shock around 1.7%. Peaches, nuts, peanuts and mustard seem to have an increased risk of a systemic reaction. As mentioned previously, patients that are not allergic to pollen seem to have an increased risk of severe reactions.
What should I do if I have pollen-food syndrome?
Since it is still a reaction mediated by antibodies, it is impossible to predict the severity of a future reaction. Therefore, it seems safer to recommend the avoidance of the non-cooked form of the fruits or vegetables that causes symptoms in a particular individual. In regards to nuts, we generally recommend that patients avoid those that are problematic, even in the cooked form. If in doubt, it might be useful to consult an allergist who will be able to do further testing, clarify the diagnosis and provide the appropriate recommendations.