Oral allergy syndrome (OAS) is an allergic reaction to fruits, vegetables and nuts caused by an individual’s similar allergy to pollen (small, powdery grains that are the fertilizing element of flowering plants, much of which can easily become airborne). The cross-reaction between the two is the result of similarities in proteins found in some types of pollen and in some fruits, vegetables and nuts. Also known as pollen-food syndrome, the reaction causes itching and swelling in the mouth, throat and lips when a certain fruit, vegetable or nut is consumed.

OAS is always cross-reactive. A nut allergy is not the same as oral allergy syndrome. However, an individual who is allergic to nuts may also have an OAS cross-reaction when eating some nuts. While both are allergic reactions, OAS and a nut allergy will often result in different symptoms.

Individuals with OAS must first suffer from hay fever (allergic rhinitis) or a similar pollen allergy. Because of the similarities between the proteins in different types of foods, this type of cross-reactivity is possible. With a pollen allergy, a person’s immune system has been sensitized to the allergen pollen. When the immune system encounters a food protein resembling the pollen protein to which it is sensitive, it triggers the allergic cascade.

Because people with pollen allergies tend to be more sensitive to pollen during the seasons when it is present in the air, OAS reactions can be stronger during this period. This time period can vary depending on the type of pollen allergy.

OAS results in itching or swelling around the mouth, lips, tongue or throat. This occurs as the fruits, vegetables or nuts are consumed and their proteins come into contact with these local areas. Some of the proteins are absorbed through the lining of the mouth and trigger the immune system to release certain chemicals that cause localized allergy symptoms. The chemicals cause the cells in the lining of the mouth and throat to swell, resulting in inflammation. Individuals may also develop symptoms from simply handling raw fruit and vegetables.

This type of allergic reaction is based on superficial contact, and the fruit, vegetable and nut proteins are not absorbed into the blood and circulated throughout the body in great quantity. This usually produces only mild symptoms. Typically, OAS symptoms cause mild discomfort for only for a few minutes before subsiding.

In some rare individuals, gastrointestinal symptoms occur after the problem fruit, vegetable or nut has passed into the digestive tract. Nausea, diarrhea and an upset stomach may all result in these individuals.

While an OAS reaction is usually mild, highly sensitive people can experience life-threatening anaphylactic shock.  This condition is characterized by a constriction of the airways that makes breathing difficult. OAS can also trigger an asthma attack in sensitive individuals.

Only fresh fruits, vegetables and nuts can cause OAS. Because their proteins are so temperature-sensitive, these foods are often less likely to cause an OAS reaction after being canned or boiled. Some allergic individuals also find that frozen fruits and vegetables do not cause an allergic reaction. Nuts, however, may trigger a reaction whether raw or cooked.