Why can I eat cooked fruit but not fresh fruit?

The proteins that cause OAS (PR-10, profilins) are heat-labile - they break down at temperatures above 60 °C. Cooking, baking or pasteurisation usually destroys their structure, so the immune system no longer reacts. That is why apple pie or stewed fruit is usually fine, while a fresh apple causes itching.

Is OAS different from a "real" food allergy?

Yes. OAS is a localised reaction in the mouth and throat; the symptoms pass quickly and are rarely dangerous. A "real" food allergy (IgE-mediated systemic allergy) can cause hives all over the body, breathing difficulties, vomiting, low blood pressure and anaphylaxis - and as a rule it does not settle with cooking. If symptoms go beyond the mouth, do not wait - see an allergist.

Can I develop OAS later in life if I never had it before?

Yes. OAS usually develops after years of sensitisation to pollen. Many patients describe having had seasonal pollen symptoms for years with no food problems, and then in their late twenties or thirties they start reacting to a fresh apple or hazelnut. The change is usually gradual and tracks the intensity of your underlying pollen allergy.

How does an allergist diagnose OAS?

A combination of patient history (which foods, which symptoms, which seasons make it worse), skin prick tests with the fresh fruit (prick-to-prick test) and specific IgE blood tests for individual proteins (Bet v 1, profilins, LTP). Commercial tests with standard extracts have lower sensitivity for OAS because commercial processing destroys the labile proteins.

Do I need an adrenaline auto-injector?

For most people with typical, mouth-only OAS symptoms - no. An auto-injector (EpiPen / Anapen / Jext) is prescribed for people who have already had a systemic reaction, who have asthma, who react to cooked food, or who are sensitised to heat-stable proteins (LTP - lipid transfer proteins, especially in peach and apple). The assessment is made by an allergist.

Are all fruit varieties equally problematic?

No. Different varieties of the same fruit contain different amounts of allergenic protein. With apples, some modern varieties (e.g. Golden Delicious, Granny Smith) often cause stronger reactions than old varieties (Boskoop, Ontario). Experimenting under an allergist's supervision can help you find the varieties you tolerate better.

Glossary

OAS

Oral allergy syndrome (Pollen-Food Allergy Syndrome)

A cross-reactive allergic reaction in which proteins from pollen react with similar proteins in fresh food - itching and swelling of the mouth after eating fruit, vegetables or tree nuts.

ICD-10: T78.1

What it is

OAS (oral allergy syndrome, Pollen-Food Allergy Syndrome) is a type of cross-reactive allergic reaction where proteins in pollen structurally resemble similar proteins in fresh food. The most common scenario in Croatia: a person allergic to birch experiences itching and swelling of the mouth after biting into a fresh apple. The reaction is usually localised to the mouth and throat, sets in within a few minutes of eating, and clears up on its own within half an hour.

How it is measured

The mechanism is structural similarity of proteins. The main allergens of birch (Bet v 1) and similar tree species are PR-10 proteins, which are evolutionarily related to proteins in the fruit of the Rosaceae family (apple, pear, peach, cherry). The immune system of people sensitised to birch recognises these proteins in food as the same threat. Another important family is the profilins - ubiquitous plant proteins that cause a broader spectrum of cross-reactivity, especially in people allergic to grasses.

Symptoms

Typical symptoms appear 5-15 minutes after eating raw food:

• Itching and tingling of the lips, tongue, palate and throat • Swelling of the lips or the lining of the mouth (angioedema limited to the mouth) • Irritation of the tonsils and throat, sometimes a feeling of "tightness" • Less often: itchy ears, sneezing, watery eyes

In 1-3% of cases the reaction can progress to a systemic allergy with hives, nausea, vomiting or a more severe reaction such as anaphylaxis. Any systemic reaction, or a reaction to cooked food, requires urgent medical assessment.

Sources and data

The most common pollen-food combinations in Croatia, according to clinical data:

• Birch → apple, pear, cherry, peach, plum, hazelnut, carrot, celery, kiwi • Ragweed → watermelon, melon, cucumber, banana, courgette • Mugwort → carrot, celery, pepper, anise, caraway, coriander • Grasses → tomato, potato, melon, watermelon, kiwi, peanut

The full profile for all Croatian allergens is available at /krizna-reaktivnost, with a list of 60+ foods sorted by reaction severity.

What you can do

Practical advice for people with confirmed OAS:

1. Cooking solves 80-90% of cases - heat-labile proteins break down above 60 °C. Apple pie, cooked apple, stewed fruit - usually no symptoms.

2. Peel the skin - in apples, pears and peaches the skin contains more allergenic protein than the flesh.

3. Different varieties sometimes help - e.g. some people tolerate old apple varieties better than the commercial Golden Delicious.

4. Be more careful during the pollen season - when your allergen is flowering heavily, cross-reactions to food can be more intense.

5. Talk to an allergist about testing - a prick test with fresh food or a specific IgE test for individual proteins (Bet v 1, profilins) clarifies your personal profile.

Context in Croatia

In Croatia, OAS is most often linked to birch allergy because of the high rate of sensitisation to the Bet v 1 protein - studies estimate that 50-70% of adults allergic to birch also develop OAS. Diagnosis is available through allergy clinics (KBC Zagreb, KBC Split, KBC Rijeka, KBC Osijek, and a number of private practices) and combines patient history, skin tests and specific IgE proteins.

Frequently asked questions

Why can I eat cooked fruit but not fresh fruit?: The proteins that cause OAS (PR-10, profilins) are heat-labile - they break down at temperatures above 60 °C. Cooking, baking or pasteurisation usually destroys their structure, so the immune system no longer reacts. That is why apple pie or stewed fruit is usually fine, while a fresh apple causes itching.
Is OAS different from a "real" food allergy?: Yes. OAS is a localised reaction in the mouth and throat; the symptoms pass quickly and are rarely dangerous. A "real" food allergy (IgE-mediated systemic allergy) can cause hives all over the body, breathing difficulties, vomiting, low blood pressure and anaphylaxis - and as a rule it does not settle with cooking. If symptoms go beyond the mouth, do not wait - see an allergist.
Can I develop OAS later in life if I never had it before?: Yes. OAS usually develops after years of sensitisation to pollen. Many patients describe having had seasonal pollen symptoms for years with no food problems, and then in their late twenties or thirties they start reacting to a fresh apple or hazelnut. The change is usually gradual and tracks the intensity of your underlying pollen allergy.
How does an allergist diagnose OAS?: A combination of patient history (which foods, which symptoms, which seasons make it worse), skin prick tests with the fresh fruit (prick-to-prick test) and specific IgE blood tests for individual proteins (Bet v 1, profilins, LTP). Commercial tests with standard extracts have lower sensitivity for OAS because commercial processing destroys the labile proteins.
Do I need an adrenaline auto-injector?: For most people with typical, mouth-only OAS symptoms - no. An auto-injector (EpiPen / Anapen / Jext) is prescribed for people who have already had a systemic reaction, who have asthma, who react to cooked food, or who are sensitised to heat-stable proteins (LTP - lipid transfer proteins, especially in peach and apple). The assessment is made by an allergist.
Are all fruit varieties equally problematic?: No. Different varieties of the same fruit contain different amounts of allergenic protein. With apples, some modern varieties (e.g. Golden Delicious, Granny Smith) often cause stronger reactions than old varieties (Boskoop, Ontario). Experimenting under an allergist's supervision can help you find the varieties you tolerate better.

OAS