It’s possible to react to an allergen you’ve never encountered before because one of its proteins is very similar to one of your triggers. This is called a cross-reaction.
Cross-reactions can happen with different plants. For instance, you could be allergic to alder pollen and get hay fever from birch, beech and oak. And if you react to one grass pollen type then it’s common to be sensitive to others too. Timothy grass, for example, is similar to other common related grasses including: Sweet Vernal, Orchard (Cocksfoot), Perennial Rye, Meadow Fescue, Kentucky Bluegrass (June Grass) and Redtop.
People with respiratory allergies can also have cross-reactions when eating certain foods for the same reason: The protein in the food is very like the allergen. This is called pollen food syndrome or oral allergy syndrome.
For example, if you’re allergic to ragweed pollen, you might get a mild, local reaction in your mouth, throat, lips or face when you eat cantaloupes, honeydew melons, watermelon, banana, cucumber, white potato and zucchini.