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.