The allergy-causing substances are called allergens. The most common allergens are pollens, molds, dust, and animal dander. Some allergens are present only during certain seasons, for example, ragweed in the fall. Others are present year-round, for example, the mites in house dust. When the lining tissues of the nose and sinuses come into contact with allergens, a chemical called histamine is released from cells in these tissues.
Histamine causes the nose lining to swell, itch, and produce excess mucus.
Common symptoms of allergic rhinitis are itchy, runny nose sneezing nasal and head congestion. Other possible symptoms include fatigue and lethargy itchy throat postnasal drainage. Your medical history is usually the basis for the diagnosis of allergic rhinitis. Knowledge of a family history of allergic problems is also helpful. However, it may take some detective work to figure out exactly what you are allergic to.