A tree nut allergy represents a hypersensitivity reaction by the immune system to specific proteins found in nuts that grow on trees. These include common varieties like walnuts, almonds, cashews, and pecans. This allergy is distinct from a peanut allergy because peanuts are biologically classified as legumes. Developing a tree nut allergy as an adult, known as adult-onset allergy, is a recognized phenomenon that can occur even after a lifetime of safely consuming these foods.
The Reality of Adult-Onset Allergies
While food allergies are often associated with childhood, a significant portion of new food allergy diagnoses occur in adults. Studies indicate that up to half of all food-allergic adults report that at least one of their allergies developed after age 18. This phenomenon challenges the older assumption that food allergies are exclusively pediatric conditions that people only outgrow.
The prevalence of adult-onset food allergies appears to be rising, with tree nuts being a common trigger alongside shellfish. This means that individuals who have always considered themselves allergy-free may suddenly find themselves managing a potentially severe new dietary restriction.
Understanding the Underlying Causes
The development of a new allergy in adulthood stems from an immune system process called sensitization, where the body mistakenly identifies a harmless food protein as a threat. This leads to the production of Immunoglobulin E (IgE) antibodies specific to the tree nut protein. Upon re-exposure, these IgE antibodies bind to immune cells, triggering the release of inflammatory chemicals that cause allergic symptoms.
One major factor is cross-reactivity, often seen in Pollen-Food Allergy Syndrome. Individuals with an existing allergy to birch pollen, for instance, may find their IgE antibodies reacting to similar proteins, specifically PR-10 proteins, found in raw tree nuts like hazelnuts. This cross-reaction can lead to mild, localized symptoms in the mouth and throat, but it can occasionally precede the development of a more systemic tree nut allergy.
The allergenic proteins that cause more severe, systemic reactions are the seed storage proteins, which belong to the cupin and prolamin superfamilies. These proteins, such as 2S albumins and 11S globulins, are highly stable and resistant to heat and digestion, allowing them to trigger a full-body reaction. Changes in the gut microbiome, known as dysbiosis, may also contribute to the loss of immune tolerance in adulthood by disrupting the microbial balance that regulates the immune system’s response to food.
Recognizing Symptoms and Seeking Diagnosis
The symptoms of a new tree nut allergy can vary widely, ranging from mild irritation to a life-threatening emergency. Mild reactions involve hives, itching or tingling in the mouth, abdominal pain, or nausea. More serious symptoms include swelling of the throat or tongue, difficulty breathing, a sudden drop in blood pressure, or dizziness, which are signs of anaphylaxis.
A formal diagnosis requires consulting an allergist who will start with a detailed clinical history of the reaction, including the timing and type of symptoms experienced. Diagnostic testing involves a skin prick test (SPT), where a small amount of tree nut extract is introduced to the skin, and a raised wheal of three millimeters or more is considered a positive result. Blood tests measure the level of tree nut-specific IgE antibodies in the serum, with a result of 0.35 kU/L or higher considered positive.
Component-Resolved Diagnostics (CRD) offers a more precise analysis by measuring IgE against specific protein components within the tree nut. Testing positive for highly stable storage proteins, such as Ana o 3 (cashew) or Cor a 9 (hazelnut), suggests a higher risk for a severe, systemic reaction. In contrast, a positive result only to a cross-reactive protein like Cor a 1 suggests a reaction more likely related to pollen allergy. A positive test indicates sensitization, not necessarily a clinical allergy, and the magnitude of the test result does not reliably predict the severity of a future reaction.
Living with a New Tree Nut Allergy
Managing a newly diagnosed tree nut allergy requires strict adherence to avoidance strategies, as there is no cure. The first step involves diligently reading the ingredient list on all food products, even for items previously considered safe, because manufacturers frequently change formulations. Tree nuts must be declared on food packaging under federal labeling laws, but advisory statements like “may contain tree nuts” are voluntary and require caution due to the risk of cross-contamination.
Anyone diagnosed with a systemic tree nut allergy must be prescribed and carry an epinephrine auto-injector. Epinephrine is the only medication that can reverse the progression of anaphylaxis and must be administered immediately if a severe reaction is suspected. An allergist will provide a written emergency action plan detailing when and how to use the auto-injector, which should be shared with family, friends, and co-workers.
Communicating the allergy is an important part of daily life, particularly when dining out. Informing restaurant staff about the allergy, asking specific questions about ingredients and preparation methods, and wearing a medical identification bracelet can significantly reduce the risk of accidental exposure. These measures, combined with professional medical guidance, allow individuals to navigate their daily lives safely despite the unexpected onset of a tree nut allergy.