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Allergic rhinitis (AR), more commonly known as hay fever, is an inflammationof the nasal passages caused by allergic reaction to airborne substances.

Allergic rhinitis is the most common allergic condition and one of the most common of all minor afflictions. It affects between 10-20% of all people in the United States, and is responsible for 2.5% of all doctor visits. Antihistamines and other drugs used to treat allergic rhinitis make up a significant fraction of both prescription and over-the-counter drug sales each year.

There are two types of allergic rhinitis: seasonal and perennial. Seasonal ARoccurs in the spring, summer, and early fall, when airborne plant pollens are at their highest levels. Perennial AR can occur at any time of year and isusually caused by home or workplace airborne pollutants. A person can be affected by one or both types. Symptoms of seasonal AR are worst after being outdoors, while symptoms of perennial AR are worst after spending time indoors.

Both types of allergies can develop at any age, although onset in childhood through early adulthood is most common. Although allergy to a particular substance is not inherited, increased allergic sensitivity may "run in the family." While allergies can improve on their own over time, they can also become worse over time.

Allergic rhinitis is a type of immune reaction, in which immune proteins called antibodies react with otherwise harmless substances. Such substances are known as allergens.

AR involves a special set of cells in the immune system known as mastcells, which are found in the lining of the nasal passages and eyelids. Mastcells display on their surface a special type of antibody, called immunoglobulin type E (IgE). When these antibodies encounter allergens, they trigger release of a variety of chemicals onto neighboring cells, including blood vessels and nerve cells. One of these chemicals, histamine, binds to the surfaces of these other cells, through special proteins called histamine receptors. Interaction of histamine with receptors on blood vessels causes neighboring cells to become leaky, leading to the fluid collection, swelling, and increased redness characteristic of a runny nose and red, irritated eyes. Histamine alsostimulates pain receptors, causing the itchy, scratchy nose, eyes, and throat common in allergic rhinitis. Because of the central role of histamine in the allergic response, antihistamine drugs are the primary weapon against allergies.

The number of possible airborne allergens is enormous. Seasonal AR is most commonly caused by grass and tree pollens. Different plants release their pollen at different times of the year, so seasonal AR sufferers may be most affected in spring, summer, or fall, depending on which plants they are most sensitive to. The amount of pollen in the air is reflected in the pollen count, often broadcast on the daily news during allergy season. Pollen counts tend to be lower after a good rain that washes the pollen out of the air and higher onwarm, dry, windy days.

Virtually any type of tree or grass may cause AR. A few types of weeds that tend to cause the most trouble for people include the following:

Perennial AR is often triggered by house dust, a complicated mixture of airborne particles, many of which are potent allergens. House dust contains some or all of the following:

House mite body parts. All houses contain large numbers of microscopic insects called house mites. These harmless insects feed on fibers, fur, and skin shed by the house's larger occupants. Their tinybody parts easily become airborne.
Animal dander. Animals constantly shed fur, skin flakes, and dried saliva. Carried in the air, or transferredfrom pet to owner by direct contact, dander can cause allergy in many sensitive people.
Mold spores. Molds live in damp spots throughout the house, including basements, bathrooms, air ducts, air conditioners, refrigeratordrains, damp windowsills, mattresses, and stuffed furniture. Mildew and other molds release airborne spores which circulate throughout the house.

Inflammation of the nose, or rhinitis, is the major symptom of AR. Inflammation causes itching, sneezing, runny nose, redness, and tenderness. AR usuallyalso causes reddened, itching, and watery eyes.

Diagnosing seasonal AR is usually easy and can often be done without a medical specialist. When symptoms appear in spring or summer and disappear with theonset of cold weather, seasonal AR is almost certainly the culprit.

Allergy tests, including skin testing and provocation testing, can help identify the precise culprit, but may not be done unless a single source is suspected and subsequent avoidance is possible. Skin testing involves placing a small amount of a specific allergen under the skin to observe whether redness and swelling occurs.

Perennial AR can also usually be diagnosed by careful questioning about the timing of exposure and the onset of symptoms. Specific allergens can be identified through allergy skin testing.

Avoidance of the allergens is the best treatment, but this is often not possible. When it is not possible to avoid one or more allergens, there are two major forms of medical treatment, drugs and immunotherapy.

The most common drugs for treating AR are called antihistamines. Antihistamines block the histamine receptors on nasal tissue, decreasing the effect of histamine release by mast cells. They may be used after symptoms appear, thoughthey may be even more effective when used preventively, before symptoms appear. A wide variety of antihistamines are available.

Most over-the-counter antihistamines may produce drowsiness as a major side effect. Newer antihistamines that do not cause drowsiness are available by prescription.

Other drugs used to treat AR include decongestants, corticosteroids, and mastcell stabilizers.

Immunotherapy, also known as desensitization or allergy shots, is also available for treatment of allergies. Immunotherapy alters the balance of antibodytypes in the body, thereby reducing the ability of IgE to cause allergic reactions. Immunotherapy is preceded by allergy testing to determine the preciseallergens responsible. Injections involve very small but gradually increasingamounts of allergen, over several weeks or months, with periodic boosters. Full benefits may take up to several years to achieve and are not seen at allin about one in five patients.

Most people with AR can achieve adequate relief with a combination of preventive strategies and treatment. While allergies may improve over time, they mayalso get worse or expand to include new allergens. Early treatment can helpprevent an increased sensitization to other allergens.

Reducing exposure to pollen may improve symptoms of seasonal AR. Strategies include the following:

Stay indoors with windows closed during the morning hours, when pollen levels are highest.
Keep car windows up whiledriving.
Use a surgical face mask when outside.
Wash clothes and hair after being outside.
Clean air conditioner filters in the home regularly.

Keep the house dry through ventilation and use of dehumidifiers.
Use a disinfectant such as dilute bleach to clean surfaces such as bathroom floors and walls.
Clean and disinfect air conditioners and coolers.

Vacuum frequently, and change the bag regularly. Use a bagwith small pores to catch extra-fine particles.
Clean floors and walls with a damp mop.

Avoid contact if possible.
Wash hands after contact.
Vacuum frequently.
Keep pets out of the bedroom, andoff furniture, rugs, and other dander-catching surfaces.
Have yourpets bathed and groomed frequently.

Hay fever