Oral allergy syndrome

Oral allergy syndrome, also known as pollen-food allergy, is an IgE-mediated hypersensitivity reaction to specific raw, plant-based foods including fruits, vegetables, nuts and certain spices. It typically presents with mild tingling or pruritus of the lips, tongue and mouth. This hypersensitivity reaction is initiated by cross-reaction with a non-food allergen, most commonly birch pollen whereby the protein in the food is similar but not identical in structure to the original allergen. This explains why OAS is strongly linked with pollen allergies and presents with seasonal variation. It also explains why cooking culprit food, and thereby denaturing the proteins, prevents symptoms occurring.

• A food allergy is caused by direct sensitivity to a protein present in food. OAS is caused by cross-sensitisation to a structurally similar allergen present in pollen.
• Non-plant foods do not cause OAS because there are no cross-reactive allergens in pollen which would be structurally similar to meat. Food allergies may be caused by plant or non-plant foods and allergens occur in response to substances that are more stable and able to survive stomach enzymes and acid processing and cooking.
• Food allergies are more likely to cause systemic symptoms (vomiting and diarrhoea) and are more likely to lead to anaphylaxis because the allergens are not readily broken down. The symptoms of OAS are normally limited to the oropharynx because the proteins that cause the symptoms are denatured by stomach enzymes.

• About 2% of the UK population has OAS but this is probably under-diagnosed.
• About 1/2 of patients with a general pollen allergy and 3/4 of patients with an allergy to birch pollen (the most common allergen in the UK) report symptoms of OAS.

• Birch pollen allergy (most common)
• Rye grass pollen allergy
• Rubber latex allergy

• OAS should be suspected in patients with a history of seasonal allergy symptoms (rhinitis, conjunctivitis and asthma) who present with symptoms minutes after eating a specific raw food.
• Itching and tingling of the lips, tongue and mouth are the most common symptoms
• Mild swelling and redness of the lips, tongue and throat may occur.
• In severe cases, patients develop nausea and vomiting.
• Symptoms fully resolve within one hour of contact.
• Anaphylaxis is a very rare complication.

• Most patients will have no visible signs during or between episodes of OAS.
• Some patients may have visible swelling and redness of the lips, tongue and oral mucosa.
• Patients with anaphylaxis present with wheeze, hives, hypotension and collapse.

• OAS is a clinical diagnosis but further tests can be used to rule out diagnoses such as food allergies and to confirm the diagnosis when the history is unclear.
• Standard IgE RAST and skin prick testing may be performed to identify common allergens such as pollens which co-exist with OAS. Note that common pollen allergies have specific associated foods which cause OAS. For example, birch allergy is associated with sensitivity to, amongst others, carrot, parsley and spinach.
• Food-specific IgE testing: this is unreliable and rarely performed clinically because the food does not survive processing.
• Skin prick testing with culprit foods is more commonly used. A positive test would cause a small, itchy lump to develop on the skin.

• Avoidance of culprit foods is the only required modification for the vast majority of patients.
• If culprit food is eaten and symptoms develop, an oral antihistamine can be taken.
• If symptoms of anaphylaxis develop, an ambulance should be called. Intramuscular adrenaline may be required.
• Patients should be informed that once cooked, culprit foods should not cause symptoms.