A tree nut allergy is a hypersensitivity to dietary substances from tree nuts and edible tree seeds, causing an overreaction of the immune system, which may lead to severe physical symptoms. Tree nuts include almonds, Brazil nuts, cashews, chestnuts, filberts/hazelnuts, macadamia nuts, pecans, pistachios, shea nuts, and walnuts. Management is by avoiding eating the causal nuts or foods that contain them among their ingredients, and a prompt treatment if there is an accidental ingestion. Total avoidance is complicated because the declaration of the presence of trace amounts of allergens in foods is not mandatory in every country. Tree nut allergies are distinct from peanut allergy, as peanuts are legumes, whereas a tree nut is a hard-shelled nut.
Signs and symptoms
Food allergies in general usually have an onset of symptoms in the range of minutes to hours for an immunoglobulin E (IgE)-mediated response, which may include anaphylaxis. Symptoms may include rash, hives, itching of mouth, lips, tongue, throat, eyes, skin, or other areas, swelling of lips, tongue, eyelids, or the whole face, difficulty swallowing, runny or congested nose, hoarse voice, wheezing, shortness of breath, diarrhea, abdominal pain, lightheadedness, fainting, nausea, or vomiting. Non-IgE-mediated responses occur hours to days after consuming the allergenic food, and are not as severe as IgE-mediated symptoms. Symptoms of allergies vary from person to person and incident to incident. Potentially life-threatening, the anaphylactic onset of an allergic reaction is characterized by respiratory distress, as indicated by wheezing, breathing difficulty, cyanosis, and circulatory impairment that can include a weak pulse, pale skin, and fainting. This can occur when IgE antibodies are released and areas of the body not in direct contact with the food allergen show severe symptoms. Untreated, the overall response can lead to vasodilation, which can be a low blood pressure situation called anaphylactic shock. Consumption of raw nuts usually causes a more severe reaction than roasted nuts or food-grade nut oils, as processing can reduce the integrity of the allergic proteins.
Causes Tree nut seed storage proteins are the allergens responsible for severe tree nut allergic reactions. Tree nut allergies are caused by an irregular immune system reaction to two kinds of nut protein – those for storage and metabolism – which bind to immunoglobulin E (IgE) antibodies, triggering an immune defense and reaction symptoms that may be mild or as severe as anaphylaxis. Storage proteins include the prolamin superfamily (associated with 2S albumins) and the cupin superfamily consisting of legumin-group proteins (the 11S globulin family) and vicilins (the 7S globulins). Additional tree nut proteins, which may be called pan-allergens, include lipid transfer proteins, profilins (involved in nut structure), pathogenesis-related proteins, and heveins, which resemble proteins in pollens and seeds that cause IgE-mediated allergic reactivity in susceptible people.
Diagnosis An allergy test or food challenge may be performed at an allergy clinic to determine the exact allergens involved and to guide a safe treatment or management plan. Healthcare providers analyze how the body reacts to allergens in the safety of a regulated setting. Since a tree nut allergy can be life-threatening, people who suspect they are having allergic reactions to any kind of tree nut should be tested by an allergist immediately. Tree nut allergies can be genetic and passed down. Skin-prick (scratch) tests and blood tests may be used to determine if an allergy is present by measuring the presence of immunoglobulin E (IgE), an antibody that responds to allergens and triggers the release of chemicals which cause the symptoms. If the test results are inconclusive an oral food challenge may be used as a final determining factor. This test consists of feeding the patient tiny amounts of the food that they believe is causing their allergic reactions. This is done under the direct supervision of the allergist. Oral food challenges are only used when a full patient history is taken and when the probability of passing the test is high.
Prevention Prevention involves an exclusion diet and vigilant avoidance of foods that may be contaminated with tree nuts, nut particles, or oils extracted from nuts. In the United States, the federal Food Allergen Labeling and Consumer Protection Act (FALCPA) requires that any packaged food product that contains tree nuts as an ingredient must list the specific tree nut on the label. Foods that almost always contain tree nuts include pesto, marzipan, Nutella, baklava, pralines, nougat, gianduja, and turrón. Other common foods that may contain tree nuts include cereals, crackers, cookies, baked goods, candy, chocolates, energy/granola bars, flavored coffee, frozen desserts, marinades, barbecue sauces, and some cold cuts, such as mortadella. Tree nut oils (especially shea nut) are also sometimes used in lotions and soaps. Asian and African restaurants, ice cream parlors, and bakeries are considered high-risk for people with tree nut allergy due to the common use of nuts and the possibility of cross contamination.
Cross-reactivity
People with clinically confirmed tree nut allergy to one type of tree nut may have cross-reactivity to other tree nut species and to peanuts, which are not nuts but rather part of the legume family. The cause is similarity in protein structures. Identifiable allergenic proteins are grouped into families: cupins, prolamins, profilins, and others. Tree nuts have proteins in these families, as do peanuts and other legumes. Reviews of human trials report that for a confirmed tree nut allergy, up to one-third of people will react to more than one type of tree nut. The cross reactivity among almond, walnut, pecan, hazelnut, and Brazil nut is stronger than the cross reactivity of these toward cashew or pistachio. People with tree nut allergy are seldom allergic to just one type of nut, and are therefore usually advised to avoid all tree nuts, even though an individual may not be allergic to the nuts of all species of trees. Someone allergic to walnuts or pecans may not have an allergy to cashews or pistachios, because the two groups are only distantly related and do not necessarily share related allergenic proteins.
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