Pollen During the spring, summer, and fall, plants release pollen (a fine powder that is carried by the wind from plant to plant in order to fertilize them). Each microscopic grain of pollen contains proteins that can cause allergic reactions when breathed in. These proteins can trigger asthma symptoms. Grasses, trees, and weeds (such as ragweed) are most often responsible for causing allergies. Back to Articles Related Articles Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor. © 1995-2021 KidsHealth®. All rights reserved. Images provided by The Nemours Foundation, iStock, Getty Images, Veer, Shutterstock, and Clipart.com.