Cross-reactivity

Cross-reactivity is a phenomenon in which the immune system reacts to several different allergen sources because they share structurally similar proteins. The best-known form is pollen-food (OAS), but cross-reactions also exist between pollens themselves (e.g. birch and alder) and between other sources (latex and fruit, dust mites and shellfish). The practical consequence: if you have a confirmed allergy to one allergen, it is worth proactively checking your risk for related sources.

The mechanism is IgE antibodies that recognise a shared "epitope" - a short part of the protein structure. The main protein families responsible for cross-reactivity in Croatia:

• PR-10 (Bet v 1 superfamily): birch, alder, hazel, ash, oak → apple, pear, peach, cherry, hazelnut, carrot, celery, kiwi • Profilins: ubiquitous in plants, linking grasses, trees, fruit and vegetables • LTP (lipid transfer proteins): heat-stable, more common in Mediterranean populations, linking peach, apple, hazelnut and olive • Polcalcins: grasses → cross-reactions within the Poaceae family

Cross-reactivity presents differently depending on the protein family and the route of exposure:

• Pollen-pollen (e.g. birch + alder + hazel): classic rhinitis and conjunctivitis symptoms - a person reacts to several tree species because they are biochemically related.

• Pollen-food (OAS): localised symptoms in the mouth after fresh fruit, vegetables or tree nuts - in detail at /pojmovnik/oas.

• Latex-fruit syndrome: cross-reactions between latex proteins and banana, avocado, kiwi, chestnut - symptoms ranging from itching to hives, sometimes systemic reactions.

• Mites-crustaceans-cockroaches: the tropomyosin protein in dust mites cross-reacts with crabs and lobsters and with cockroaches - systemic reactions are possible.

A detailed visual overview of all cross-reactions by Croatian allergen (21 allergens × 60+ foods) is available at /krizna-reaktivnost, with tables of severity levels (frequent / occasional / rare reaction). The data are based on the Sampson 2014 clinical studies, EAACI/WAO guidelines and local Croatian allergy registries.

1. Get tested for component allergens - classic extract tests show "birch allergy" but do not reveal which exact protein is involved. Specific IgE to Bet v 1, profilin, LTP, etc. clarifies your profile and predicts your risk for food.

2. Keep a diary of your own reactions - the season, the allergen, the foods, the symptoms. Patterns only become visible after a few months.

3. Strictly avoid any food that has already triggered a systemic reaction in you - each next one can be stronger.

4. Allergist assessment for an auto-injector - people sensitised to LTP (heat-stable proteins) have a higher risk of systemic reactions than people with "only" PR-10 sensitisation.

5. Talk to your doctor before immunotherapy - SLIT/SCIT for birch can also affect cross-reactive foods.

Cross-reactivity is the main reason the range of "pollen allergy" is so broad in Croatia - a person with primary sensitisation to birch often develops reactions to 5-10 foods and 3-4 additional types of pollen over their lifetime. A systematic approach (test → diary → personalised avoidance) is far more effective than reactively cutting individual foods out of the diet.

What are protein families and why do they matter?

The proteins that cause allergies are grouped into "families" according to structural similarity. The best known in Croatia are Bet v 1 (PR-10) from birch and the profilins. An immune system that reacts to one protein from a family will, as a rule, also react to other related ones - that is why birch allergy also causes a reaction to apple, pear, hazelnut, carrot and celery.

Can I develop new cross-allergies over my lifetime?

Yes. Cross-reactivity can develop or change over the years. People with a pollen allergy can develop new food sensitivities, especially if pollen exposure increases or the underlying pollen allergy worsens. Track your reactions and talk to an allergist about any changes.

Do I need to cut out all the food listed in the cross-reactivity tables?

No. The tables are a list of possible cross-reactions, not of every reaction you will have. Most patients with a birch allergy react to only a few of the 10 possible foods, not all of them. Cut out only what actually causes you symptoms - the rest of the food is safe and nutritionally valuable.

When should I see an allergist?

If your symptoms go beyond mild itching in the mouth, if they are getting worse, if they affect your quality of life, if you have respiratory symptoms (cough, wheeze), or if you are not sure whether it is OAS or a true food allergy. Testing helps clarify your profile and gives you a personal avoidance plan.

Does immunotherapy help with cross-reactions?

Partly. Immunotherapy (SLIT/SCIT) for the primary allergen (e.g. birch) reduces cross-reactions to food in some studies, but results vary. Immunotherapy is primarily prescribed for pronounced pollen symptoms, with the secondary effect on food being an added benefit - talk to an allergist about realistic expectations in your case.

• OAS
• Allergy season

• Cross-reactive allergies - why your mouth tingles from an apple
• The ABC of allergens - what allergens are and how they react

• EAACI Molecular Allergology User's Guide
• alergija.hr - Complete profile of cross-reactions by allergen