Oral Allergy Syndrome Explained

Oral allergy syndrome (OAS) is the most common food allergy in adults and often affects people who also have seasonal allergies to substances like birch, ragweed, grass pollen, mugwort, alder, or latex. [2] In fact, researchers estimate that between 47% and 70% of people with pollen allergies may also experience OAS. [2]

OAS develops because some proteins found in raw fruits, vegetables, nuts, and other plant-based foods are structurally similar to proteins found in certain pollens. [1] This phenomenon is known as molecular mimicry. As a result, the immune system may mistake proteins in food for pollen allergens and trigger an allergic reaction. [1]

This immune response is referred to as cross-reactivity. [3] When someone with pollen allergies is exposed to a triggering allergen, the immune system produces antibodies called immunoglobulin E (IgE). These antibodies attach to immune cells throughout the body and can trigger the release of histamine, which contributes to inflammation and allergy symptoms. [4]

Because of cross-reactivity, these IgE antibodies may also react to similar proteins found in certain raw fruits, vegetables, nuts, and seeds. [1] The specific foods that may trigger symptoms often depend on the type of pollen allergy a person has:

• Birch – Apples, pears, peaches, apricots, cherries, plums, nectarines, prunes, kiwi, carrots, celery, potatoes, peppers, fennel, parsley, coriander, parsnips, hazelnuts, almonds, and walnuts
• Ragweed – Bananas, melons, zucchini, cucumber, dandelions, and chamomile tea
• Grass pollen – Peaches, celery, melons, tomatoes, and oranges
• Mugwort – Celery, apples, kiwi, peanuts, fennel, carrots, parsley, coriander, sunflower seeds, and peppers
• Alder – Celery, pears, apples, almonds, cherries, hazelnuts, peaches, and parsley
• Latex – Bananas, avocado, kiwi, chestnuts, and papaya, which contain proteins similar to those found in latex

Unlike non-IgE-mediated food allergies, OAS has the potential to cause anaphylaxis, a severe allergic reaction that may affect breathing and requires immediate medical attention. [5] However, severe reactions are relatively uncommon. Fewer than 2% of people with OAS experience anaphylaxis, and most food allergy symptoms are generally mild and limited to the mouth or throat. [2]

An OAS allergic reaction can occur seconds or minutes after consuming a potential cross-reactive food. In other words, it develops quickly. [2]

Once the immune system recognizes the cross-reactive protein, it'll trigger a localized immunoinflammatory response in the mouth, lips, tongue, and throat. Individuals with OAS symptoms may experience: [2]

• Itchy mouth
• Scratchy throat
• Tingling
• Swelling
• Bumps

Less common symptoms include: [2]

• Nausea
• Hives
• Rash
• Difficulty breathing
• Anaphylaxis
• Itchy, swelling, or red skin where you touched the foods

Typically, OAS symptoms resolve within a few hours, though the duration of a food allergy reaction can vary in severity and presentation between individuals, and may even vary for the same person depending on the level of allergen exposure. [2]

An OAS diagnosis is typically based on clinical history, in addition to a physical examination and allergy testing. In patients with a history of airborne pollen allergies, itching and tingling of the mouth after eating fresh, raw fruits, vegetables, or nuts often points to OAS. [6]

During a physical exam, a healthcare provider will examine the patient’s mouth and throat for signs of swelling or redness. The exam is typically coupled with an allergy test to identify certain allergens, in the form of:

• Skin prick tests – The skin is lightly pricked with a suspected allergen. The patient is then observed by a healthcare provider to identify signs of an allergic reaction. Results are often immediate.
• Blood tests – Slightly more invasive, a healthcare provider will extract blood from the patient and examine the levels of IgE antibodies, after exposing the patient to certain food-based proteins. Blood tests are also available as at-home lab tests that you can conduct within your own home. Whether performed at home or in a medical office, results will typically take up to a few days.

A healthcare provider may also administer a food challenge test, in which you’ll be given increasing amounts of certain foods and observed for signs of an allergic reaction. [2]

Unfortunately, there is no designated treatment for OAS. Rather, effective management of OAS involves a combination of strategies that aim to prevent or minimize symptoms and reduce the risk of complications. These can involve: [2]

• Avoiding potential trigger foods – After undergoing diagnostic allergen tests, most individuals will understand what types of food may trigger an OAS reaction. As such, the most effective way to manage OAS symptoms is to avoid allergens completely to evade potential cross-reactivity.2 If you have seasonal allergies, it’s generally recommended to avoid these trigger foods during these times, as your reaction may be more severe than usual. [2]
• Immunotherapy – Some healthcare providers may administer allergy shots or sublingual immunotherapy to those with severe OAS reactions or multiple pollen allergies. Over time, individuals are exposed to small amounts of the allergen to help desensitize the immune system to certain pollens and food proteins. [7]
• Antihistamine medication – Over-the-counter antihistamines can block some of the effects of histamine in the body to reduce the severity of an allergic reaction and reduce inflammation in the mouth, tongue, lips, and throat. [8] Antihistamine medications include Benadryl, Allegra, and Claritin.
• Epinephrine – Some individuals may carry around an injectable epinephrine pen if they’re prone to severe allergic reactions or anaphylaxis. Epinephrine, also known as adrenaline, helps to relax the muscles and open the airways. [2]

Generally, it’s unlikely that children under the age of three will experience cross-reactivity, or OAS. Typically, it takes a few years to develop a pollen or food allergy. [2] Also, it’s not unusual for adults to develop pollen or food allergies later in life—particularly if they’ve moved to a new geographical location with vegetation they haven’t yet been exposed to. [9]

So, how exactly do pollen allergies develop?

Allergy development involves two phases: [9]

• Sensitization – When your body first encounters an allergen, like ragweed, the body will begin making IgE antibodies to defend itself against the new, and potentially harmful, substance. During this phase, however, you’re unlikely to experience an allergic reaction.
• Reaction – The second time your body encounters an allergen, it’ll take action. The IgE antibodies bind to the allergen and other immune cells to produce histamine and other chemicals that trigger inflammation and other allergy symptoms throughout the body. Within minutes, your mouth, tongue, lips, and throat may begin to tingle or itch.

See related: Are Food Allergies Genetic?

Oral allergy syndrome is often confused with other types of food allergies. However, food allergies and OAS typically cause different symptoms. While signs of OAS are generally limited to the mouth, tongue, lips, and throat, food allergies can trigger a full-body allergic reaction.

Condition	What Causes It	Common Symptoms	Additional Notes
Oral Allergy Syndrome (OAS)	Cross-reactivity between pollen allergens and proteins found in certain raw fruits, vegetables, nuts, and seeds	Symptoms are typically limited to the mouth, lips, tongue, and throat	Most common in people with seasonal pollen allergies
IgE-Mediated Food Allergy	Triggered by immunoglobulin E (IgE) antibodies in the immune system	Dizziness Lightheadedness Itchy skin Raised skin Rash Swelling Coughing Wheezing Trouble breathing Sneezing Abdominal pain Diarrhea [10]	Can cause a full-body allergic reaction and, in some cases, anaphylaxis
Non-IgE-Mediated Food Allergy	Triggered by T cells in the immune system rather than IgE antibodies [12]	Nausea Vomiting Diarrhea Dehydration Weight loss Lethargy Abdominal pain Gassiness Lack of appetite Constipation Bloating Shock [11]	Symptoms are typically isolated to the gastrointestinal tract, although some people may experience chronically itchy or blistered skin, such as with celiac disease. [11] These food allergies may also develop into food hypersensitivity syndromes, including: [11] Food protein-induced enterocolitis syndrome (FPIES) Allergic proctocolitis (AP) Food protein-induced enteropathy (FPE) Celiac disease (specific to gluten allergy)
Food Intolerance	Occurs when the digestive system has difficulty breaking down certain foods	Diarrhea Bloating Gassiness Stomach pain [13]	Unlike food allergies, food intolerances do not involve an immune system response and are rarely life-threatening.

When the immune system wrongly recognizes a food protein as an allergen, it will disperse IgE to bind to cells throughout the body and initiate an allergic response. In the case of OAS, symptoms are typically isolated to the mouth, throat, tongue, and lips. However, other types of food allergies can impact the whole body.

The at-home Food Allergy Test from Everlywell will measure your body’s IgE reactivity to common food allergens, like almonds, cow’s milk, wheat, and more.