You’re allergic to birch pollen. You eat an apple. Your mouth itches. Why?

Your body identifies allergens as intruders, releasing chemicals that cause an allergic reaction: itching and swelling. Allergic reactions to pollen typically start in your nose. Allergic reactions to food typically start in your mouth. Both can progress into gastrointestinal distress or anaphylaxis.

As components of plants, pollen and produce often have similar protein structures. If a protein in a fruit or vegetable is similar to a pollen protein that you’re allergic to, it will cause a reaction inside your mouth like the reaction the pollen would cause inside your nose.

This is called cross-reactivity, or pollen-food allergy syndrome. People who are allergic to apples and carrots tend to experience cross-reactivity with birch pollen because all three contain similar proteins. Ragweed pollen and melons are also cross-reactive. Cooking the food that’s causing a cross-reaction usually solves the problem, as heat breaks down the proteins your body identifies as allergens.

An antihistamine will help alleviate cross-reaction symptoms in the short term. If you have food allergies, you should always carry an epinephrine autoinjector.

Allergies worsen over time. Avoid foods that trigger reactions.

Consult your allergist with questions.