Oral allergy syndrome (OAS) is also called “pollen-food allergy syndrome,” “pollen-food syndrome,” and “pollen-associated food allergy syndrome”. It is among the most common types of food allergy.
It is an allergic mouth reaction that follows the eating of fresh fruits, vegetables or nuts. This is considered to be the result of structural similarity between the proteins found in pollen and those found in fruits and vegetables. The Ige-dependent immune reaction that occurs in OAS results in allergic symptoms. Its characteristic is that it is restricted to the mucous membranes of the mouth, lips, tongue and throat.
Adults who have allergic rhinitis are more likely to suffer from OAS than other individuals.
Mechanism of OAS
OAS is a type I allergic reaction, mediated by immunoglobulin E (IgE). Initial sensitization to inhaled pollen is followed by a cross-reaction to the antigens in some food, which produces the characteristic features.
The proteins usually responsible for OAS include proteases, α-amylase inhibitors, peroxidases, profilins, seed-storage proteins, thiol proteases, and lectins in plants. In animal foods, they include muscle proteins, enzymes, and various serum proteins.
OAS is usually a mild disease. Patients with OAS may start to show symptoms within a few minutes of eating the culprit food. These include itching, a burning sensation on the mouth and lips, as well as the ear or throat. The area around the mouth may swell, or urticarial rashes may appear all over the body. Other symptoms occur in the eyes, nose and skin. Inflammation of the tongue or uvula may even lead to a suffocating feeling. Anaphylaxis is also possible, though rare. Breathing difficulty, drop in blood pressure or a rash on the skin is uncommon but possible.
OAS is classified based on the severity of the symptoms:
- (I) confined to the oral mucosa
- (II) symptoms in both the oral mucosa and gastrointestinal tract
- (III) symptoms in the oral mucosa accompanied by systemic symptoms (e.g. asthma, conjunctival congestion or rhinitis, urticaria, angioedema)
- (IV) potentially fatal symptoms as well as those of the oral mucosa (e.g laryngeal edema, shock)
The difference in symptoms may depend on the kind of protein involved.
Diagnosis and Treatment
Diagnosis depends on the symptoms followed by food testing, of which skin prick testing using fresh food is the most sensitive. Other tests include in vitro tests and provocation tests. Blood tests for this condition include RIST (Radioimmunosorbent Test) which determines total IgE, and RAST (Radioallergosorbent Test) which assays the IgE antibodies to a particular allergen.
Other diseases with similar symptoms include angioedema and burning mouth syndrome.
Treatment depends on avoiding the trigger food or thermal processing of such foods. Antihistamines are required to abort or treat an attack. Topical mast cell stabilizers, such as cromolyn sodium, are also useful in some patients. If the patient desires, self-injectable epinephrine (such as EpiPen, 0.3 mg epinephrine in 0.3 mL) may be made available for the immediate treatment of life-threatening symptoms. Immunotherapy has not produced conclusive results.