Oral allergy syndrome (OAS) is an allergic reaction to food that involves itchiness, redness and swelling of the mouth after eating certain fruits, vegetables or nuts. The condition is a type of allergic cross-reaction, where the body reacts to one substance after mistaking it for another. The condition occurs as a result of the immune system being unable to distinguish between proteins in pollen (which cause allergies such as hay fever) and similar proteins in some foods.

While OAS and a nut allergy are both types of an allergic response that can occur from nuts, the two are not the same. Symptoms are usually different, and an individual can have both conditions, or one or the other. However, a person must have an allergy to pollen (even a very mild one) in order to be affected by OAS.

OAS only causes a reaction when fresh fruits, vegetables or nuts are consumed. Because the proteins in these foods are temperature sensitive, heating or freezing these foods often makes them far less allergenic. An individual may be more sensitive to OAS during those seasons when problem pollen allergens are in the air.

OAS is usually not a severe condition, and medical treatment is rarely necessary. Symptoms usually include only slight discomfort that lasts for a matter of minutes. However, more sensitive individuals can react with gastrointestinal symptoms (e.g., nausea, diarrhea) or, in extreme cases, with the life-threatening condition known as anaphylactic shock. Reactions can also trigger asthma attacks in individuals with asthma.

Antihistamines are commonly used to treat the symptoms associated with OAS. These drugs are capable of disrupting the processes of the allergic reaction and lessening symptoms.

Allergy shots are also effectively used as a preventative treatment against OAS. This type of immunotherapy involves regular injections that build up a patient’s resistance to certain allergens. Patients with OAS who receive allergy shots for pollen proteins usually find their reactions to fruits, vegetables and nuts subside as well. However, allergy shots are intended to work for pollen allergies, not food allergies.