The signs and symptoms are familiar to many:
- Sneezing often accompanied by a runny or clogged nose
- Coughing and postnasal drip
- Itching eyes, nose, and throat
- Allergic shiners (dark circles under the eyes caused by increased blood flow near the sinuses)
- The "allergic salute" (in a child, persistent upward rubbing of the nose that causes a crease mark on the nose)
- Watering eyes
- Conjunctivitis (an inflammation of the membrane that lines the eyelids, causing red-rimmed, swollen eyes, and crusting of the eyelids).
In people who are not allergic, the mucus in the nasal passages simply moves foreign particles to the throat, where they are swallowed or coughed out. But something different happens to a person who is sensitive to airborne allergens.
As soon as the allergen lands on the mucous membranes lining the inside of the nose, a chain reaction occurs that leads the mast cells in these tissues to release histamine and other chemicals. These powerful chemicals contract certain cells that line some small blood vessels in the nose. This allows fluids to escape, which causes the nasal passages to swell, resulting in nasal congestion.
Histamine also can cause sneezing, itching, irritation, and excess mucus production, which can result in allergic rhinitis (runny nose). Other chemicals made and released by mast cells, including cytokines and leukotrienes, also contribute to allergic symptoms.
SOURCE: National Institute of Allergies and Infections Diseases, National Institutes of Health publication: Something in the Air: Airborne Allergens