Why Hazelnut Allergy Behaves Differently in Türkiye

Hazelnut allergy is not a single condition, it has two mechanisms that differ in severity. What makes it interesting is that which one dominates depends on **where you live**.

Two kinds of hazelnut allergy

**Primary hazelnut allergy.** The immune system reacts directly to hazelnut proteins. Reactions can affect the digestive, respiratory or cardiovascular systems, and may include bronchospasm or even anaphylaxis.

**Oral allergy syndrome (OAS).** This is linked to pollen allergy. Because hazelnut proteins resemble pollen proteins, the immune system confuses them. Symptoms are usually confined to the mouth: itching, tingling and swelling of the lips and throat.

The proteins responsible

Several allergenic proteins have been identified in hazelnuts:

**Cor a 1**, a PR-10 protein of 17 kDa and the major allergen. It generally causes mild symptoms, that is OAS. It is **heat-sensitive**, cooking or roasting disrupts its structure.

**Cor a 8, Cor a 9, Cor a 11, Cor a 14**, storage proteins and heat-stable allergens, associated with primary hazelnut allergy and more serious reactions.

The distinction matters in practice: someone sensitised to Cor a 1 may react to raw hazelnuts but tolerate roasted ones. For someone sensitised to Cor a 9 or Cor a 14, heat treatment offers no protection.

The birch pollen connection

Birch pollen allergy is very common in northern Europe. Its major allergen is **Bet v 1**.

Cross-reactivity between Cor a 1 and Bet v 1 is reported at **over 97%**, with about 67% sequence homology.

The result: someone allergic to birch pollen may feel itching in the mouth after eating hazelnuts. Among 161 hazelnut-sensitised patients in the Netherlands, **86% were sensitised to Cor a 1** and **87% to Bet v 1**, a very strong correlation (Spearman coefficient 0.98).

In an Austrian study of 35 birch-pollen-allergic patients, cross-sensitisation reached **71%**.

Why is the picture different in Türkiye?

Here is the most interesting part. The prevalence of Cor a 1 sensitisation falls as regional exposure to Bet v 1 and homologous pollen allergens decreases.

In one study, **only 4 of 55 hazelnut-sensitised patients in Türkiye (7.3%)** had detectable IgE to Cor a 1.

Compare that with 86% in the Netherlands.

The reason is geographical. Birch is widespread in northern Europe but not in most of Türkiye. In a population not exposed to birch pollen, the associated cross-sensitisation does not develop.

The practical implication is that hazelnut allergies seen in Türkiye may not be predominantly mild OAS as in Europe, but distributed differently, a difference worth considering in diagnosis.

How common is it?

The worldwide prevalence of confirmed tree nut allergy ranges from **0% to 1.6%**.

Hazelnut is the **most common tree nut allergy in Europe**. According to a systematic review, it accounts for between 17% and 100% of tree nut allergies in Europe, with wide regional variation.

In a study of 731 patients across 12 European centres, reaction severity was distributed as follows: **mild 47.9%, moderate 36%, severe 15.9%.** Anaphylaxis occurred in 3% of patients.

When to see a doctor

Assessment is needed if any of the following follow eating hazelnuts or hazelnut products: itching and tingling in the mouth, swelling of the lips, tongue or throat, skin redness or hives, nausea and vomiting, shortness of breath or wheezing, dizziness.

Difficulty breathing, trouble swallowing or faintness is an emergency, call emergency services immediately.

How is it diagnosed?

An allergist takes a history and may order skin prick testing or specific IgE blood tests. Molecular diagnosis, identifying which protein you react to, helps with risk assessment.

Self-testing or trial and error is dangerous. Home challenges must not be attempted, particularly with a history of serious reactions.

Reading labels

For people with allergies, hazelnut is among the allergens that must be declared on labels. It is usually printed in bold or listed separately.

Phrases such as "may contain traces of hazelnut" or "produced in a facility handling hazelnuts" indicate cross-contamination risk. Those with serious allergies should avoid such products.

Hazelnuts can appear unexpectedly in chocolate, desserts, ice cream, granola and some sauces.

Important note

This article is for information. If you suspect an allergy, consult an allergy and immunology specialist. Diagnosis and treatment decisions belong with your doctor.

Sources

ThermoFisher Phadia Allergen Encyclopedia, "f17 Hazelnut" and "f428 Cor a 1" monographs.

Biedermann T. et al., "Birch pollen allergy in Europe", Allergy, 2019.

"Patterns of Clinical Reactivity in a Danish Cohort of Tree Nut Allergic Children, Adolescents, and Young Adults", Frontiers in Allergy, 2022.

Severity assessment of hazelnut allergy in 731 patients across 12 European centres.

Study on the prevalence of Cor a 1 IgE among hazelnut-sensitised patients in Türkiye.