Many common substances can be allergens. Pollens, food, mold, dust, feathers, animal dander, chemicals, drugs such as penicillin, and environmental pollutants commonly cause many to suffer allergic reactions.
Pollens

One of the most significant causes of allergic rhinitis in the United States is ragweed. It begins pollinating in late August in most of the U.S. and continues until the first frost. Late springtime pollens come from grasses like timothy, orchard, red top, sweet vernal, Bermuda, Johnson, and some bluegrasses. Early springtime hay fever is most often caused by pollens of trees such as elm, maple, birch, poplar, beech, ash, oak, walnut, sycamore, cypress, hickory, pecan, cottonwood, and alder. Flowering plants rarely cause allergy symptoms.

Learn more about common plant allergens in this area >>
Household allergens