Why can pollen from oaks and birches cause allergic reactions?

Pollen from trees in the order Fagales—which includes oaks and birches—triggers allergic reactions because these plants rely on the wind for cross-fertilization. To ensure successful reproduction, wind-pollinated plants produce vast quantities of lightweight pollen that can easily become airborne and travel long distances.1

When an allergic person inhales this pollen, their immune system mistakenly identifies the harmless particles as a threat. Upon initial exposure, the body becomes sensitized by developing specific antibodies. During subsequent exposures, the allergen reacts with these antibodies on the surface of mast cells and basophilic leukocytes. This triggers the release of histamine and other substances into the tissues. Histamine irritates mucus-secreting glands and blood vessels, leading to the characteristic symptoms of hay fever, such as sneezing, nasal congestion, and itchy eyes.21