A hazelnut allergy is a common form of tree nut allergy, affecting individuals across all age groups. It is particularly prevalent in Europe due to high consumption. The allergic reaction occurs when the immune system mistakenly identifies certain hazelnut proteins as a threat, releasing Immunoglobulin E (IgE) antibodies. Hazelnut allergy is categorized into two main types: a primary allergy, which causes severe, systemic reactions, and a secondary allergy often linked to seasonal hay fever. Primary allergies involve sensitization to heat-stable storage proteins found within the nut kernel.
Physical Reactions to Hazelnuts
Ingesting hazelnuts can trigger responses ranging from mild irritation to life-threatening systemic reactions. Mild symptoms often appear rapidly, involving the skin or digestive system. These reactions include raised, itchy patches of skin (hives), general skin flushing, or the worsening of pre-existing atopic dermatitis.
Moderate reactions frequently involve the gastrointestinal tract, causing abdominal pain, nausea, vomiting, or diarrhea. Swelling of the lips, tongue, or face (angioedema) may also occur, indicating a widespread immune response.
The most severe reaction is anaphylaxis, a rapid, multi-system response that impairs breathing and circulation. Respiratory distress manifests as shortness of breath, wheezing, or a constricted feeling in the throat. Anaphylaxis can also cause a sudden drop in blood pressure, leading to dizziness, fainting, and shock. This systemic reaction is associated with the highly stable hazelnut storage proteins, Cor a 9 and Cor a 14.
The Link to Pollen Allergies
Many allergic reactions to hazelnuts are not due to a primary nut allergy but rather Pollen Food Allergy Syndrome (PFAS), also called Oral Allergy Syndrome (OAS). This cross-reactivity occurs when the immune system confuses hazelnut proteins with those found in common environmental allergens. The most frequent connection is between the Cor a 1 protein in hazelnuts and the Bet v 1 protein found in birch pollen.
Due to this protein similarity, a person allergic to birch pollen may react when eating raw hazelnuts. PFAS symptoms are usually confined to the mouth and throat, manifesting as tingling, itching, or a scratchy sensation. This localized reaction occurs because the allergenic proteins are easily broken down by heat and digestive enzymes.
Individuals with PFAS often safely consume cooked, roasted, or baked hazelnuts. Heat processing denatures the Cor a 1 protein, altering its structure so the immune system no longer recognizes it. In contrast, people with a primary hazelnut allergy react to both raw and cooked nuts because their causative proteins, Cor a 9 and Cor a 14, are highly resistant to heat and digestion. This distinction is important as PFAS reactions are generally mild, while primary allergies carry a higher risk for systemic anaphylaxis.
Identifying a Hazelnut Allergy
Diagnosis of a hazelnut allergy begins with a detailed medical history taken by a qualified allergist, reviewing the patient’s symptoms and reaction timing. Skin Prick Tests (SPT) are a common initial diagnostic tool. This involves placing hazelnut extract on the skin and lightly pricking the surface; a raised, red welt indicates IgE antibodies reactive to hazelnut proteins.
Blood tests measure the concentration of hazelnut-specific IgE antibodies (sIgE). Component-Resolved Diagnosis (CRD) is a specialized analysis that measures IgE levels to specific proteins (Cor a 1, Cor a 9, and Cor a 14). Testing these components helps distinguish between the milder PFAS (Cor a 1) and the more severe primary allergy (Cor a 9 and Cor a 14).
The Oral Food Challenge (OFC) remains the definitive method for confirming a food allergy diagnosis. During an OFC, the patient consumes gradually increasing amounts of the suspected allergen under strict medical supervision. This controlled environment ensures any reaction can be immediately treated and provides the most accurate confirmation of a clinical allergy. An OFC is typically performed when initial tests are inconclusive or when there is suspicion the patient may have outgrown the allergy.
Treatment and Safe Avoidance
Managing a diagnosed hazelnut allergy centers on strict avoidance of the nut in all forms. This requires careful attention to food labels, as hazelnuts are often used in baked goods, chocolate, cereals, and as ingredients in flavorings. Reading ingredient lists for the word “hazelnut” or other names like “filbert” is necessary to prevent accidental exposure.
Patients must also be cautious about “May Contain” warnings, which indicate a risk of cross-contamination during manufacturing. Preventing cross-contamination is important in the home kitchen and when dining out, as tiny amounts of the protein can trigger a reaction. Using separate utensils, cutting boards, and cooking surfaces can help minimize this risk.
For emergency treatment, individuals diagnosed with a systemic allergy must carry an auto-injectable epinephrine device at all times. Epinephrine is the medication of choice for treating anaphylaxis because it rapidly reverses airway swelling and dangerously low blood pressure. Patients and caregivers must be trained to administer epinephrine immediately at the first signs of a severe reaction, such as difficulty breathing or lightheadedness. After administering the epinephrine, emergency services must be called, as the patient requires observation to manage the potential for a secondary or biphasic reaction.