Why does OAS occur? Step 1: You become sensitised to pollen Your immune system develops an allergic response to pollen from trees, grasses or weeds. Step 2: You eat a cross-reactive food A raw apple, for example, contains proteins similar to certain birch-pollen proteins. Step 3: Your immune system reacts It recognises the food protein as if it were pollen, triggering an allergic reaction in your mouth and throat. This can cause itching, tingling or mild swelling. Source: Cleveland Clinic and Allergy Australia 2D Representation of Common Pollen Allergen Proteins Types of Oral Allergy Syndrome (OAS) Pollens Birch Pollen Pollen Name Birch Pollen Allergens Bet v 1, a PR-10 protein Folded protein made of amino acids, not a small molecule chemical structure Source Birch trees (Betula species) Common Foods Associated Apple, pear, peach, cherry, plum, kiwi, carrot, celery, hazelnut, almond Grass Pollen Pollen Name Grass Pollen Allergens Phl p 1 and Phl p 5; Phl p 12 (profilin) Contributes to cross-reactivity with foods Source Grasses, including timothy grass (Phleum pratense) Common Foods Associated Melon, watermelon, tomato, potato, kiwi, orange, peanut Ragweed Pollen Pollen Name Ragweed Pollen Allergens Amb a 1 and profilin Amb a 8 Source Ragweed weeds (Ambrosia species) Common Foods Associated Banana, cucumber, watermelon, cantaloupe, other melons, zucchini, and sometimes sunflower seeds. Share this article