A tree nut allergy is an abnormal immune response to one or more tree nuts. When a person with an allergy is exposed to certain tree nuts, their immune system launches an “attack” against the proteins in the tree nut and can even create antibodies (specifically immunoglobulin E or IgE) that target the nut proteins. This response provokes the rapid development of allergic symptoms, which can affect a variety of body systems and may be mild, moderate, or severe and life-threatening.

There are many different kinds of tree nuts, including almonds, Brazil nuts, cashews, hazelnuts, pecans, pistachios, pine nuts and walnuts. An allergy to one tree nut does not guarantee an allergy to another, but it is common for people to react to more than one tree nut. Tree nut allergies tend to develop during childhood and are usually lifelong.^[1]

An allergic reaction to tree nuts can cause a variety of signs and symptoms that range from mild to severe and may be systemic (affecting the whole body) or localized (affecting a single body part or organ). Reactions may include skin involvement (e.g., rashes, swelling, itching); eye discomfort (e.g., itchy, watery, and/or swollen eyes); respiratory difficulties (e.g., shortness of breath, coughing, wheezing, throat swelling, hoarseness); gastrointestinal issues (e.g., nausea, vomiting, cramping, diarrhea); cardiovascular changes (e.g., drops in blood pressure, increased heart rate); and neurological responses, although rare (e.g., fainting, seizures, dizziness).

Most signs and symptoms occur within minutes of exposure to tree nuts, although a delayed response of up to 2 hours is possible. Exposure to tree nuts usually happens through ingestion — skin contact or inhalation rarely causes an allergic reaction. When the allergic reaction is very severe and/or involves more than one organ system, it is called an anaphylactic reaction, which can be life-threatening without prompt treatment.^[2][3]

A history of consistent allergic signs and symptoms after the ingestion of a tree nut is the first step in diagnosing a tree nut allergy. An allergist will use this history in conjunction with a physical exam to guide the choice of additional diagnostic tests.

The skin prick test is the most common diagnostic test and involves introducing the allergen into the skin through a small prick with a special tool. If IgE antibodies to tree nuts are present, the area that was pricked will swell. A swollen area 3 mm or greater in size is usually considered positive for an allergy, although some clinicians prefer an 8 mm or greater standard to decrease the chance of a false positive.^[4] Blood tests that detect food-specific IgE antibodies may also be used to aid in the diagnosis.

Although these diagnostic tests are helpful, they do not predict the severity of a person’s allergic reaction, nor are they useful in people without a history of allergic signs and symptoms (given the frequency of false positives). In some cases, an oral food challenge, which consists of gradual consumption of the allergen under direct medical supervision, may be recommended to confirm the allergy.^[5][6] Ideally, allergy testing is done yearly because some allergies change or resolve over time.

Avoiding exposure to tree nuts is the first line of treatment for a tree nut allergy. This includes not eating foods that may be contaminated with tree nuts.^[5]

When exposure does occur, medications can be used to treat allergic signs and symptoms. If the reaction is mild, antihistamines may be helpful, but severe reactions should be treated with epinephrine.^[7] Prompt treatment is critical in preventing life-threatening reactions. For this reason, most people with a tree nut allergy are prescribed an epinephrine auto-injector and are instructed to carry it with them at all times in case of an emergency.

Vitamin D supplementation is an area of ongoing research for the prevention of food allergies. Vitamin D is integral to the proper functioning of the immune system, and vitamin D insufficiency and limited sun exposure are associated with the development of food allergies in observational research.^{[8][9][10][11]} However, whether vitamin D supplementation actually decreases the incidence of food allergies is not clear.^[12]

Currently, there is also no evidence to support the use of supplements in treating tree nut allergies. However, there is ongoing research investigating the effects of certain Chinese herbs on the immune system, including their potential to dampen the inappropriate immunologic response to food proteins.^[13] Of these herbs, berberine appears to be the most powerful suppressor of IgE production.^[14] More research is needed to determine whether these supplements could decrease allergic reactions.

Diet is the cornerstone of tree nut allergy management. People with tree nut allergies need to carefully plan their diets to avoid eating certain tree nuts. This means also avoiding foods that contain or may be contaminated with tree nuts, such as food products made in facilities that use nuts in other products, which is a potential source of cross-contamination. Most countries require that food labels clearly state whether a food contains tree nuts, and many labels voluntarily state whether cross-contamination is possible, so checking the label on all packaged food items is important for people with allergies.

Diet may even play a role in allergy prevention. Early introduction of allergenic foods (by approximately 6 months of age) alongside continued consumption reduces the risk of developing food allergies.^[15][16][17]

Allergen avoidance and medications for accidental exposure are the mainstays of tree nut allergy management. Other treatments, such as immunotherapy and monoclonal anti-IgE antibodies, are under investigation for their efficacy in treating food allergies, including tree nut allergy. The main goal of these immune-modulating therapies is to allow a person with a food allergy to be exposed to their allergen without experiencing a severe reaction.^[18][19]

Tree nut allergies are caused by an inappropriate reaction of the immune system to the proteins found in tree nuts. Normally, the immune system targets and attacks pathogens like viruses and bacteria but in the case of food allergies, the immune system identifies a certain food as an “invader” and launches an immune response against it.

This breakdown of immune tolerance is primarily driven by T helper 2 cells and often includes the production of food-specific IgE antibodies, all of which lead to a cascade of immune responses (like the activation of mast cells and release of cytokines) that cause allergic symptoms. Why the immune system reacts abnormally to certain foods in some people is not fully understood.^[20]