Hazelnut’s Place Among Tree Nuts
Hazelnut is definitively classified as a tree nut. The common hazel (Corylus avellana) belongs to the family Betulaceae, firmly placing it within the category of true nuts. Tree nuts are a major group of food allergens, distinct from peanuts, which are legumes. The presence of hazelnut must be clearly labeled on packaged foods under regulations like the Food Allergen Labeling and Consumer Protection Act (FALCPA) in the United States. A diagnosis of an allergy to one tree nut often prompts a recommendation to avoid all tree nuts due to the common grouping and potential for cross-reactivity. Hazelnut allergy is one of the most prevalent tree nut allergies, particularly in Europe, and frequently triggers severe reactions.
Systemic Reactions to Hazelnut
A systemic reaction to hazelnut represents the most severe form of this allergy. These reactions are triggered by the immune system releasing Immunoglobulin E (IgE) antibodies against hazelnut proteins. They are generalized, affecting multiple body systems, and typically occur rapidly, often within minutes of ingestion. Symptoms include hives (urticaria), a widespread itching sensation on the skin, and angioedema (swelling of the face, lips, tongue, or throat). Respiratory distress manifests as wheezing, coughing, shortness of breath, or a tight feeling in the chest; gastrointestinal symptoms like abdominal pain, vomiting, and diarrhea may also occur. The most dangerous systemic reaction is anaphylaxis, a life-threatening condition marked by a sudden drop in blood pressure, shock, and severe difficulty breathing that requires immediate emergency treatment with an epinephrine auto-injector.
Oral Allergy Syndrome and Hazelnut
Hazelnut is strongly associated with Oral Allergy Syndrome (OAS), also known as Pollen-Food Allergy Syndrome. This condition is a result of cross-reactivity, where the immune system, already sensitized to a specific airborne pollen, mistakes similar proteins in the hazelnut for the pollen allergen. The link is primarily with birch pollen, as the major birch pollen allergen (Bet v 1) shares a similar structure with the hazelnut protein Cor a 1. People with a history of hay fever due to birch pollen are the most likely to experience OAS when consuming raw hazelnuts.
Symptoms of OAS are typically mild and localized to the mouth and throat, appearing immediately upon contact with the food. Reactions include an itchy sensation in the mouth, tongue, or ears, a scratchy throat, and sometimes mild swelling of the lips. Unlike systemic allergies, OAS symptoms rarely progress to a full-body reaction or anaphylaxis because the implicated proteins are fragile and easily broken down by saliva and stomach acids. This fragility also explains why roasting or cooking the hazelnut often mitigates the allergic reaction, as the heat denatures the cross-reactive proteins.
Diagnosis and Necessary Avoidance
Diagnosis of a hazelnut allergy begins with a detailed clinical history, followed by objective testing. Common diagnostic tools include skin prick tests, where a small amount of hazelnut extract is introduced just beneath the skin to observe for a localized wheal or bump. Blood tests measure the level of hazelnut-specific Immunoglobulin E (IgE) antibodies in the serum, which helps confirm the presence of an IgE-mediated allergy. In cases where the initial tests or clinical history are inconclusive, a supervised oral food challenge remains the gold standard for definitive diagnosis, performed in a medical setting to confirm whether a person reacts to the food.
Management of a confirmed hazelnut allergy centers on strict dietary avoidance. Hazelnut can be a hidden ingredient in flavorings, baked goods, chocolates, and spreads. Individuals diagnosed with a systemic hazelnut allergy must carry an epinephrine auto-injector at all times and be trained on its immediate use in case of accidental exposure. Consulting with an allergist is essential for a personalized management plan, which may also include identifying and avoiding sources of potential cross-contamination.