Pollen is a fine powder produced by certain plants when they reproduce. During
the spring, summer, and fall seasons, it's released into the air and picked up by
the wind, which brings it to other plants to fertilize them. Inside of these pollen
grains are proteins that commonly cause allergic reactions (such as sneezing, runny
nose, and itchy eyes) when breathed in. The pollen that's most often responsible for
causing allergies comes from grasses, trees, and weeds. Many people with asthma are
allergic to pollen. When they breathe it in, it can trigger their asthma symptoms.