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What is oral allergy syndrome (OAS)?

Oral allergy syndrome (OAS) is an allergy that occurs in people with hay-fever due to a cross reaction between foods and the tree, grass or weed pollen that they are allergic to. It is usually milder than a typical immediate type food allergy with symptoms that occur only in the mouth when eating certain foods, usually fresh fruits, nuts and vegetables.

OAS is common, occurring in 60-70% of adult birch allergic patients with hayfever. These people have antibodies to pollen that also cross-react with proteins found in certain foods (see what is an allergy).

  • Birch allergic patients typically react to apple, cherry, celery, peach, pear, carrot or apricot
  • Rye grass allergic patients may react to melon, peanut, tomato or watermelon
  • Ragweed allergic patients react to melon, zucchini or banana
  • Latex allergic patients may react to kiwi, banana, avocado, strawberry, chestnut and papaya

OAS symptoms can occur at any time but are more common during the pollen season.

  • Itch and/or swelling of mouth and throat that starts within minutes or seconds of ingesting the food
  • It is rare (but not impossible) for OAS to progress to systemic more severe symptoms

The diagnosis of OAS is usually made with the typical history of the specific foods involved. In uncertain cases or to check for other foods involved skin prick tests, blood tests or oral food challenges may be required.

  • Treating hayfever with antihistamines, nasal spray or immunotherapy (see allergic rhinitis brochure) will help with those symptoms but will not prevent the cross reactive OAS symptoms.
  • People with OAS should avoid those specific foods. In some cases they may be able to eat the food if it is cooked or peel
  • If symptoms suddenly occur, spit out the food, rinse the mouth with water and take an oral antihistamine
Download our “Oral allergy syndrome” leaflet for free