Many people with a birch pollen allergy notice that they also react to certain foods: a tingling or itching sensation in the mouth after eating a raw apple, a swollen lip from hazelnuts, or a tickle in the throat from carrot. This is called cross-allergy (also known as oral allergy syndrome or pollen-food syndrome). An estimated 50--70% of all birch pollen allergy sufferers are affected.
Key point: the allergen Bet v 1 in birch pollen resembles proteins in many types of fruit, nuts and vegetables. Your immune system mistakes these food proteins for birch pollen and triggers an allergic reaction. Cooking the food breaks down these proteins -- apple sauce often causes no symptoms at all.
How does cross-allergy develop?
The allergen in birch pollen is called Bet v 1. This protein is structurally very similar to proteins found in many types of fruit, nuts and vegetables. The immune system recognises these food proteins as "birch pollen" and responds with the same allergic reaction. Hence the term cross-allergy: the immune system cross-reacts between two allergens that actually come from different sources.
Which foods cause cross-reactions?
Fruit
Apple is the most common trigger. Also: pear, cherry, peach, apricot, plum, nectarine, kiwi and blackberry.
Nuts
Hazelnut is, alongside apple, the most commonly reported cross-reaction. Almond, walnut and peanut can also cause reactions.
Vegetables
Carrot and celery are the most important. Also: tomato, potato and soybeans.
Herbs & spices
Anise, cumin, coriander and parsley also contain proteins that resemble Bet v 1.
What symptoms do you get?
Symptoms occur almost immediately after eating the food (within a few minutes) and are usually limited to the mouth and throat:
- Tingling, itching or burning sensation in the mouth
- Swelling of lips, tongue or palate
- Tickle or lump in the throat
- Sometimes itchy ears
Please note: in rare cases a more serious allergic reaction can occur. If you experience symptoms beyond the mouth and throat (breathlessness, hives, dizziness), seek immediate medical help.
Cooking helps
The Bet v 1-like proteins in fruit and vegetables are heat-labile: they break down when heated. This means that many people who cannot tolerate raw apple can eat apple sauce, apple pie or cooked apple without any symptoms. The same applies to cooked carrot, fried tomato and pasteurised fruit juice.
Note about nuts: the allergenic proteins in hazelnut and walnut are partly heat-stable. Roasted nuts can therefore still cause symptoms.
When are the symptoms worst?
Cross-allergy symptoms are generally worst during and shortly after the birch pollen season (March--May). During this period the immune system is already activated by inhaled birch pollen and reacts more strongly to cross-reactive foods. In autumn and winter the same foods can sometimes be tolerated or cause fewer symptoms.
Tips for managing cross-allergy
Avoid raw
Avoid trigger foods in their raw form, especially during the birch pollen season.
Try cooked
Test whether you can tolerate the food when cooked: apple sauce, soup with carrot, poached pear.
Keep a diary
Note which foods cause symptoms -- this varies from person to person.
Consider immunotherapy
Desensitisation for birch pollen can in some patients also reduce cross-allergy.