The Early Show medical correspondent Dr. Emily Senay explains that seasonal allergies are caused when the immune system overreacts to irritants in the environment, such as plant and tree pollen or mold spores. Each season brings different irritants. In the fall, weeds are usually the biggest problem, from late August through October.
Ragweed, pigweed, plantain, sheep sorrel, sagebrush, and other weeds produce enormous amounts of the pollen that plagues allergy sufferers. Molds and late-blooming trees can also cause allergies in the fall.
Parts of the country with a wet summer season can experience more weed pollen and mold growth fueled by the moisture.
Allergy symptoms typically begin in early childhood, and often peak between the ages of 20 to 40.
The likelihood that a person will have an allergic reaction depends on a variety of factors, including family history.