Cross-reactivity (cross-allergy)
An allergic reaction triggered by a different allergen that is structurally close to the original one, as between pollens and certain fruits.
Cross-reactive allergy occurs when an immune system sensitised to one allergen reacts to another allergen with similar proteins that it “mistakes” for the first. It is a phenomenon of molecular resemblance.
A fresh angle (pollen-food syndrome): the most common and fascinating example is oral allergy syndrome (OAS), also known as pollen-food syndrome. Someone allergic to a pollen may react to raw fruit and vegetables whose proteins resemble that pollen:
- Birch ↔ apple, pear, cherry, peach, hazelnut, carrot, celery
- Mugwort ↔ celery, carrot, spices, fennel
- Ragweed ↔ melon, banana, courgette
- Latex ↔ banana, avocado, kiwi, chestnut (latex-fruit syndrome)
Typical OAS symptoms: tingling or itching of the mouth, lips and throat, just after eating the raw food.
The cooking nuance (reassuring): the proteins responsible for OAS are often sensitive to heat. That is why a person reacts to raw apple but tolerates stewed apple or apple pie (cooked). The same goes for raw versus cooked carrot. A major practical point that often means you can keep eating the food in cooked form.
Variable severity: OAS is generally mild (limited to the mouth), but in rare cases a cross-reactive allergy can be more severe (especially latex-fruit, or certain heat-resistant proteins). Advice from an allergy specialist is useful if in doubt.
An important distinction: cross-reactive allergy is a true allergy (IgE-mediated), not to be confused with intolerances. It explains why allergies sometimes “spread” to new foods over time.
In practice: identify the associations (with an allergy specialist), note whether symptoms occur with the raw food but not the cooked, and adapt accordingly.