The signs and symptoms of airborne allergies are familiar to many.

• Sneezing, often with a runny or clogged nose
• Coughing and postnasal drip
• Itching eyes, nose, and throat
• Watering eyes
• Conjunctivitis
• "Allergic shiners" (dark circles under the eyes caused by increased blood flow near the sinuses)
• "Allergic salute" (in a child, persistent upward rubbing of the nose that causes a crease mark on the nose)

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 in a person who is sensitive to airborne allergens.

In sensitive people, as soon as the allergen lands on the lining inside the nose, a chain reaction occurs that leads the mast cells in these tissues to release histamine and other chemicals. The 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.

Some people with allergy develop asthma, which can be a very serious condition. The symptoms of asthma include

• Coughing
• Wheezing
• Shortness of breath