Oral allergy syndrome (OAS) or pollen-food allergy is a type of food allergy classified by a cluster of allergic reactions in the mouth and throat in response to eating certain (usually fresh) fruits, nuts, and vegetables that typically develops in adults with hay fever. OAS is not a separate food allergy, but rather represents cross-reactivity between distant remnants of tree or weed pollen still found in certain fruits and vegetables. Therefore, OAS is only seen in people with seasonal pollen allergies, and mostly people who are allergic to tree pollen. It is usually limited to ingestion of uncooked fruits or vegetables. In adults, up to 60% of all food allergic reactions are due to cross-reactions between foods and inhalative allergens. OAS is a Type 1 or immunoglobulin E-mediated hypersensitivity, which is sometimes called a "true allergy". The body's immune system produces IgE antibodies against pollen; in OAS, these antibodies also bind to (or cross-react with) other structurally similar proteins found in botanically related plants. OAS can occur any time of the year, but is most prevalent during the pollen season. Individuals with OAS usually develop symptoms within minutes of eating the food. Signs and symptoms Individuals with OAS may have any of a number of allergic reactions that usually occur very rapidly, within minutes of eating a trigger food. The most common reaction is an itching or burning sensation in the lips, mouth, ear canal, or pharynx. Sometimes other reactions can be triggered in the eyes, nose, and skin. Swelling of the lips, tongue, and uvula, and a sensation of tightness in the throat may be observed. Once the allergen reaches the stomach, it is broken down by the acid, and the allergic reaction does not progress further. The natural course of the disease is that it usually persists. If an affected person swallows the raw food, and the allergen is not destroyed by the stomach acids, it is likely that there will be a reaction from histamine release later in the gastrointestinal tract. Vomiting, diarrhea, severe indigestion, or cramps may occur. Rarely, OAS may be severe and present as wheezing, vomiting, hives, low blood pressure, or anaphylaxis. Development, causes A person with an allergic disposition is sensitized to allergenic pollen proteins through the respiratory route can develop an allergy to heat labile food proteins (for example profilins) in certain fruits and vegetables, which cross-react with pollen proteins. OAS produces