In addition, a significant percentage of people with hay fever have or develop asthma. In part, this is because trees, grasses, and weeds produce pollens during different seasons. For example, people in the eastern and Midwestern United States who are sensitive to tree pollen may suffer in the early spring when trees such as elm, maple, birch and poplar are producing pollen. People who are sensitive to pollens produced by grasses may suffer in the late spring, and early summer, the time when most grasses are pollinating. About half of all hay fever sufferers are sensitive to grass pollens.
Weeds flourish in most parts of the country from midsummer to late fall.
In the late fall, ragweed is the most common problem. In fact, ragweed is the plant that causes the most hay fever. These antibodies fix to other specialized cells throughout the body that contain powerful defensive substances such as histamine. This in turn causes the dilation of blood vessels, increased secretions of fluids, swelling of tissues, itching, sneezing, and other reactions that add up to hay fever. Removing the cause of the reaction results in immediate relief.