Birch, and the fruit that comes with it
Every spring, a fair number of people discover that a raw apple makes their lips tingle and their throat itch, and quietly conclude they have developed an apple allergy. Usually they have not. What they have is a birch allergy wearing a disguise.
The protein your immune system is actually looking at
The main allergen in birch pollen is a small protein called Bet v 1. Your immune system does not recognise "birch"; it recognises a shape. And that shape is not unique to birch: Bet v 1 belongs to a family of plant proteins, the PR-10 group, whose members look strikingly similar to each other.
Several of those relatives live in food. Apple has Mal d 1, cherry has Pru av 1, peach has Pru p 1, hazelnut has Cor a 1, carrot has Dau c 1, celery has Api g 1, soy has Gly m 4. To antibodies trained on Bet v 1, these read as near enough the same thing. So the reaction fires, in the first tissue the food touches: the mouth.
Clinically this is called pollen food allergy syndrome, or oral allergy syndrome. It is the most common food allergy in adults in northern and central Europe, precisely because birch sensitisation is so common there.
What it feels like
The classic picture is fast, local, and mild:
- Itching or tingling of the lips, tongue, palate, or throat
- A scratchy or slightly swollen feeling in the mouth
- Onset within minutes of eating, fading within about an hour of stopping
- Confined to the mouth and throat, without spreading
The usual suspects, if you react to birch: apple, cherry, peach, plum, apricot, pear, hazelnut, almond, raw carrot, celery, and kiwi. Not everyone with a birch allergy reacts to any of them, and almost nobody reacts to all of them.
This is where it stops being an article and starts being a doctor's job. Symptoms beyond the mouth, throat tightness, difficulty swallowing or breathing, hives spreading across the body, vomiting, or faintness are not oral allergy syndrome, and they need proper medical assessment rather than a blog post. Same if reactions are getting stronger each time. Polly is a consumer app and nothing here is medical advice.
Why cooking usually fixes it
Bet v 1 and its food relatives are fragile. Heat unfolds them, and once the shape is gone the antibodies no longer recognise it. Stomach acid and digestive enzymes do the same thing, which is why the reaction stays in the mouth: by the time the food is swallowed, the allergen is mostly already destroyed.
That has a practical upshot that surprises people. Baked apple, apple sauce, apple in a cake, cherry jam, cooked carrot, and roasted hazelnut are often perfectly tolerated by someone who cannot eat the same food raw. Pasteurised juice is usually fine; fresh-pressed juice frequently is not.
Two caveats worth knowing:
- Soy drink is the notable exception. Gly m 4 in unheated soy beverages has caused reactions well beyond the mouth in birch-allergic people, including severe ones. It does not behave like the fruit group.
- A tingling mouth is not always PR-10. Peach skin and hazelnut also carry sturdier allergens (the lipid transfer protein family) that survive both heat and digestion. These are far more common around the Mediterranean and can cause systemic reactions. If cooking does not help, that is a reason to get tested rather than to experiment.
Why it gets worse in April
Food reactions in this group tend to track the pollen season. During and just after birch flowering, when your immune system is already loaded with the pollen it is primed against, the threshold for a food reaction drops. The same apple that was fine in November becomes a problem in April. If you have noticed that pattern and assumed you were imagining it, you were not.
The corollary is that a food you react to in spring may well be edible for the rest of the year. Whether it is worth testing that is a question for an allergist who knows your history, not for you and an apple in a kitchen.
What to do about it
- Get the sensitisation confirmed. Component-resolved blood testing can distinguish Bet v 1 driven reactions from the sturdier allergen families, and that distinction changes how carefully you need to treat the food. It is exactly the kind of thing that is unknowable from symptoms alone.
- Note what you ate on bad days. The pollen season link is easy to miss over a single spring and obvious over two.
- Do not eliminate foods speculatively. Cutting out a long list of fruit and nuts on suspicion tends to cost more, nutritionally and socially, than it gains. Narrow it down properly.
- Ask about immunotherapy. Birch allergen immunotherapy is prescribed for the pollen symptoms themselves; effects on the associated food reactions are variable and not the reason it is given. Worth discussing if your spring is bad enough.
Where Polly fits
Polly tracks birch as its own species rather than folding it into a generic tree pollen figure, so you can see when the season starts, peaks, and ends in your location. The diary is the useful half here: log the days your mouth reacted, and the birch curve alongside them usually makes the connection visible within a single season.
Related reading: why your allergy app says "low" on a terrible day.
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