Oral allergy syndromeOccurs when proteins of certain foods are similar in structure to allergenic proteins in certain types of pollen. Patients with a pollen allergy experience symptoms of oral allergy syndrome (OAS) in response to eating certain fresh fruits, vegetables and nuts. Symptoms usually include mild swelling and itching of the lips, tongue, inside of the mouth, soft palate, throat and ears. is an IgE-mediated allergic reaction that occurs after eating certain foods. It is typically triggered by plant proteins that cross-react with certain inhaled antigens. The syndrome, sometimes called pollen-fruit syndrome, is caused by a cross-reactionCross-reactions can occur between inhaled pollen and ingested food allergens. A primary sensitization to a pollen allergen, e.g. Birch pollen can sometimes lead to oral allergy symptoms when eating apples, due to the structural similarity of the apple allergen Mal d 1 to the Birch pollen Bet v 1. of allergens found in pollen as well as in raw fruits, vegetables and some tree nuts.

Here is an example: It is possible for patients who are allergic to birch pollen to develop oral allergy syndrome when eating apples. This is a so-called cross-reaction caused by the structural similarity of birch and apple allergens and leads to a tingling, itchy feeling in the mouth. Typically, the symptoms described above occur immediately after eating the food or within 5 minutes at the latest.

Certain pollen are known to frequently cross-react with certain fruits, vegetables, and nuts. Patients allergic to Birch pollen often experience reactions when eating apples, almonds, carrots, celery, cherries, hazelnuts, kiwis, peaches, pears, and plums. Patients allergic to grass pollen often react to celery, melons, oranges, peaches, and tomatoes. Likewise, ragweed pollen can cross-react with bananas, cucumbers, melons, sunflower seeds, and zucchini.  Additionally, similarities in dust mite and cockroach allergens and seafood allergens cause similar cross-reactions.