What is Cross-Reactive Allergy?
Cross-reactive allergy occurs when IgE antibodies produced against one allergen also recognise structurally similar proteins in unrelated allergens. The immune system's antibodies do not distinguish between the original sensitising allergen and a different substance that happens to have a similar protein shape.
The most clinically important example in the UK is birch pollen cross-reactivity — sensitisation to the birch pollen protein Bet v 1 causes IgE that also recognises homologous proteins (PR-10 proteins) in a range of plant foods, producing reactions that appear unpredictable without understanding the underlying mechanism.
Oral Allergy Syndrome (Pollen-Food Allergy Syndrome)
Oral allergy syndrome (OAS) is the most common manifestation of cross-reactive food allergy. It presents with itching, tingling, or mild swelling of the lips, tongue, and throat immediately after eating certain raw fruits, vegetables, or nuts. Symptoms typically resolve within minutes and rarely progress beyond the mouth.
A key feature of OAS is that cooking eliminates the reaction — because the cross-reactive pollen-homologous proteins are heat-labile and denatured by cooking. This means that cooked apple, for example, does not cause symptoms in birch-sensitised individuals who react to raw apple.
Common Pollen-Food Pairs
| Pollen sensitisation | Common cross-reactive foods | OAS triggered by cooking? |
|---|---|---|
| Birch (Bet v 1) | Apple, pear, cherry, peach, plum, nectarine, kiwi, carrot, celery, hazelnut, almond | No — cooking destroys the cross-reactive protein |
| Grass pollen | Peach, watermelon, tomato, orange | Usually not |
| Mugwort | Celery, carrot, coriander, anise, fennel, pepper | Sometimes persists |
Lipid Transfer Protein (LTP) Allergy
LTP allergy is a more serious type of plant food allergy involving heat-stable proteins (Pru p 3 in peach is the marker allergen). Unlike OAS, LTP allergy:
- Can cause reactions to cooked foods, not just raw
- Can cause systemic reactions — hives, vomiting, asthma, or anaphylaxis
- Is more common in Southern European countries, though cases occur across the UK
ALEX3 tests specific LTP components, distinguishing this more severe allergy type from the milder birch-related OAS.
Why component testing matters: A whole-extract apple RAST test may be positive in both OAS (mild, manageable) and primary apple allergy with LTP sensitisation (potentially severe). Component testing distinguishes between them and guides very different management advice.
How ALEX3 Explains Cross-Reactive Patterns
ALEX3 tests IgE against specific molecular components, including:
- Bet v 1 (birch PR-10 protein) — confirms birch pollen as the primary sensitising source
- Homologous PR-10 food proteins (Mal d 1 in apple, Api g 1 in celery, Cor a 1 in hazelnut)
- LTP markers (Pru p 3, Ara h 9)
- Cross-reactive carbohydrate determinants (CCDs) — a common cause of false-positive IgE results
By mapping your IgE profile at the component level, ALEX3 can explain cross-reactive patterns and distinguish true primary food allergy from pollen-related sensitisation — which significantly changes the advice your GP can give you.