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.

OAS vs food allergies:

• 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.

Epidemiology:

• 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.

Associations:

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

PC

• 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.