Pollen allergy, commonly called hay fever, is one of the most common chronic diseases in the United States. Worldwide, airborne dust causes the most problems for people with allergies. The respiratory symptoms of asthma, which affects approximately 15 million Americans, are often provoked by airborne allergens (substances that cause an allergic reaction).

Overall, allergic diseases are among the major causes of illness and disability in the United States, affecting as many as 40 to 50 million Americans. The National Institute of Allergy and Infectious Diseases, a component of the National Institutes of Health, conducts and supports research on allergic diseases. The goals of this research are to provide a better understanding of the causes of allergy, to improve the methods for diagnosing and treating allergic reactions, and eventually to prevent allergies. This booklet summarizes what is known about the causes and symptoms of allergic reactions to airborne allergens, how these reactions are diagnosed and treated, and what medical researchers are doing to help people who suffer from these allergies.

Normally, the immune system functions as the body's defense against invading agents such as bacteria and viruses. In most allergic reactions, however, the immune system is responding to a false alarm. When an allergic person first comes into contact with an allergen, the immune system treats the allergen as an invader and mobilizes to attack. The immune system does this by generating large amounts of a type of antibody (a disease-fighting protein) called immunoglobin E, or IgE. Each IgE antibody is specific for one particular allergenic (allergy-producing) substance. In the case of pollen allergy, the antibody is specific for each type of pollen: one type of antibody may be produced to react against oak pollen and another against ragweed pollen, for example.

These IgE molecules are special because IgE is the only class of antibody that attaches tightly to the body's mast cells, which are tissue cells, and to basophils, which are blood cells. When the allergen next encounters its specific IgE, it attaches to the antibody like a key fitting into a lock, signaling the cell to which the IgE is attached to release (and in some cases to produce) powerful inflammatory chemicals like histamine, cytokines, and leukotrienes. These chemicals act on tissues in various parts of the body, such as the respiratory system, and cause the symptoms of allergy.

Some people with allergy develop asthma. The symptoms of asthma include coughing, wheezing, and shortness of breath due to a narrowing of the bronchial passages (airways) in the lungs, and to excess mucus production and inflammation. Asthma can be disabling and sometimes can be fatal. If wheezing and shortness of breath accompany allergy symptoms, it is a signal that the bronchial tubes also have become involved, indicating the need for medical attention.

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.

Each spring, summer, and fall, tiny particles are released from trees, weeds, and grasses. These particles, known as pollen, hitch rides on currents of air. Although their mission is to fertilize parts of other plants, many never reach their targets. Instead, they enter human noses and throats, triggering a type of seasonal allergic rhinitis called pollen allergy, which many people know as hay fever or rose fever (depending on the season in which the symptoms occur). Of all the things that can cause an allergy, pollen is one of the most widespread. Many of the foods, drugs, or animals that cause allergies can be avoided to a great extent; even insects and household dust are escapable. Short of staying indoors when the pollen count is high--and even that may not help--there is no easy way to evade windborne pollen.

People with pollen allergies often develop sensitivities to other troublemakers that are present all year, such as dust mites. For these allergy sufferers, the "sneezin' season" has no limit. Year-round airborne allergens cause perennial allergic rhinitis, as distinguished from seasonal allergic rhinitis.

Plants produce microscopic round or oval pollen grains to reproduce. In some species, the plant uses the pollen from its own flowers to fertilize itself. Other types must be cross-pollinated; that is, in order for fertilization to take place and seeds to form, pollen must be transferred from the flower of one plant to that of another plant of the same species. Insects do this job for certain flowering plants, while other plants rely on wind transport.

The types of pollen that most commonly cause allergic reactions are produced by the plain-looking plants (trees, grasses, and weeds) that do not have showy flowers. These plants manufacture small, light, dry pollen granules that are custom-made for wind transport. Samples of ragweed pollen have been collected 400 miles out at sea and 2 miles high in the air. Because airborne pollen is carried for long distances, it does little good to rid an area of an offending plant--the pollen can drift in from many miles away. In addition, most allergenic pollen comes from plants that produce it in huge quantities. A single ragweed plant can generate a million grains of pollen a day.

The chemical makeup of pollen is the basic factor that determines whether it is likely to cause hay fever. For example, pine tree pollen is produced in large amounts by a common tree, which would make it a good candidate for causing allergy. The chemical composition of pine pollen, however, appears to make it less allergenic than other types. Because pine pollen is heavy, it tends to fall straight down and does not scatter. Therefore, it rarely reaches human noses.

Among North American plants, weeds are the most prolific producers of allergenic pollen. Ragweed is the major culprit, but others of importance are sagebrush, redroot pigweed, lamb's quarters, Russian thistle (tumbleweed), and English plantain.

Grasses and trees, too, are important sources of allergenic pollens. Although more than 1,000 species of grass grow in North America, only a few produce highly allergenic pollen. These include timothy grass, Kentucky bluegrass, Johnson grass, Bermuda grass, redtop grass, orchard grass, and sweet vernal grass. Trees that produce allergenic pollen include oak, ash, elm, hickory, pecan, box elder, and mountain cedar.

It is common to hear people say that they are allergic to colorful or scented flowers like roses. In fact, only florists, gardeners, and others who have prolonged, close contact with flowers are likely to become sensitized to pollen from these plants. Most people have little contact with the large, heavy, waxy pollen grains of many flowering plants because this type of pollen is not carried by wind but by insects such as butterflies and bees.

One of the most obvious features of pollen allergy is its seasonal nature--people experience it symptoms only when the pollen grains to which they are allergic are in the air. Each plant has a pollinating period that is more or less the same from year to year. Exactly when a plant starts to pollinate seems to depend on the relative length of night and day--and therefore on geographical location--rather than on the weather. (On the other hand, weather conditions during pollination can affect the amount of pollen produced and distributed in a specific year.) Thus, the farther north you go, the later the pollinating period and the later the allergy season.

A pollen count, which is familiar to many people from local weather reports, is a measure of how much pollen is in the air. This count represents the concentration of all the pollen (or of one particular type, like ragweed) in the air in a certain area at a specific time. It is expressed in grains of pollen per square meter of air collected over 24 hours. Pollen counts tend to be highest early in the morning on warm, dry, breezy days and lowest during chilly, wet periods. Although a pollen count is an approximate and fluctuating measure, it is useful as a general guide for when it is advisable to stay indoors and avoid contact with the pollen.

Along with pollens from trees, grasses, and weeds, molds are an important cause of seasonal allergic rhinitis. People allergic to molds may have symptoms from spring to late fall. The mold season often peaks from July to late summer. Unlike pollens, molds may persist after the first killing frost. Some can grow at subfreezing temperatures, but most become dormant. Snow cover lowers the outdoor mold count dramatically but does not kill molds. After the spring thaw, molds thrive on the vegetation that has been killed by the winter cold.

In the warmest areas of the United States, however, molds thrive all year and can cause year-round (perennial) allergic problems. In addition, molds growing indoors can cause perennial allergic rhinitis even in the coldest climates.

There are thousands of types of molds and yeast, the two groups of plants in the fungus family. Yeasts are single cells that divide to form clusters. Molds consist of many cells that grow as branching threads called hyphae. Although both groups can probably cause allergic reactions, only a small number of molds are widely recognized offenders.

The seeds or reproductive particles of fungi are called spores. They differ in size, shape, and color among species. Each spore that germinates can give rise to new mold growth, which in turn can produce millions of spores.

When inhaled, microscopic fungal spores or, sometimes, fragments of fungi may cause allergic rhinitis. Because they are so small, mold spores may evade the protective mechanisms of the nose and upper respiratory tract to reach the lungs.

In a small number of people, symptoms of mold allergy may be brought on or worsened by eating certain foods, such as cheeses, processed with fungi. Occasionally, mushrooms, dried fruits, and foods containing yeast, soy sauce, or vinegar will produce allergic symptoms. There is no known relationship, however, between a respiratory allergy to the mold Penicillium and an allergy to the drug penicillin, made from the mold.

Molds can be found wherever there is moisture, oxygen, and a source of the few other chemicals they need. In the fall they grow on rotting logs and fallen leaves, especially in moist, shady areas. In gardens, they can be found in compost piles and on certain grasses and weeds. Some molds attach to grains such as wheat, oats, barley, and corn, making farms, grain bins, and silos likely places to find mold.

Hot spots of mold growth in the home include damp basements and closets, bathrooms (especially shower stalls), places where fresh food is stored, refrigerator drip trays, house plants, air conditioners, humidifiers, garbage pails, mattresses, upholstered furniture, and old foam rubber pillows.

Bakeries, breweries, barns, dairies, and greenhouses are favorite places for molds to grow. Loggers, mill workers, carpenters, furniture repairers, and upholsterers often work in moldy environments.

Like pollens, mold spores are important airborne allergens only if they are abundant, easily carried by air currents, and allergenic in their chemical makeup. Found almost everywhere, mold spores in some areas are so numerous they often outnumber the pollens in the air. Fortunately, however, only a few dozen different types are significant allergens.

In general, Alternaria and Cladosporium (Hormodendrum) are the molds most commonly found both indoors and outdoors throughout the United States. Aspergillus, Penicillium, Helminthosporium, Epicoccum, Fusarium, Mucor, Rhizopus, and Aureobasidium (Pullularia) are also common.

Similar to pollen counts, mold counts may suggest the types and relative quantities of fungi present at a certain time and place. For several reasons, however, these counts probably cannot be used as a constant guide for daily activities. One reason is that the number and types of spores actually present in the mold count may have changed considerably in 24 hours because weather and spore dispersal are directly related. Many of the common allergenic molds are of the dry spore type--they release their spores during dry, windy weather. Other fungi need high humidity, fog, or dew to release their spores. Although rain washes many larger spores out of the air, it also causes some smaller spores to be shot into the air.

In addition to the effect of day-to-day weather changes on mold counts, spore populations may also differ between day and night. Day favors dispersal by dry spore types and night favors wet spore types.

Fungi or microorganisms related to them may cause other health problems similar to allergic diseases. Some kinds of Aspergillus may cause several different illnesses, including both infections and allergy. These fungi may lodge in the airways or a distant part of the lung and grow until they form a compact sphere known as a "fungus ball." In people with lung damage or serious underlying illnesses, Aspergillus may grasp the opportunity to invade the lungs or the whole body.

In some individuals, exposure to these fungi also can lead to asthma or to a lung disease resembling severe inflammatory asthma called allergic bronchopulmonary aspergillosis. This latter condition, which occurs only in a minority of people with asthma, is characterized by wheezing, low-grade fever, and coughing up of brown-flecked masses or mucus plugs. Skin testing, blood tests, X-rays, and examination of the sputum for fungi can help establish the diagnosis. Corticosteroid drugs are usually effective in treating this reaction; immunotherapy (allergy shots) is not helpful.

Rather than a single substance, so-called house dust is a varied mixture of potentially allergenic materials. It may contain fibers from different types of fabrics; cotton lint, feathers, and other stuffing materials; dander from cats, dogs, and other animals; bacteria; mold and fungus spores (especially in damp areas); food particles; bits of plants and insects; and other allergens peculiar to an individual home.

House dust also contains microscopic mites. These mites, which live in bedding, upholstered furniture, and carpets, thrive in summer and die in winter. In a warm, humid house, however, they continue to thrive even in the coldest months. The particles seen floating in a shaft of sunlight include dead dust mites and their waste-products. These waste-products, which are proteins, actually provoke the allergic reaction.

Waste products of cockroaches are also an important cause of allergy symptoms from household allergens, particularly in some urban areas of the United States.

Household pets are the most common source of allergic reactions to animals. Many people think that pet allergy is provoked by the fur of cats and dogs. But researchers have found that the major allergens are proteins secreted by oil glands in the animals' skin and shed in dander as well as proteins in the saliva, which sticks to the fur when the animal licks itself. Urine is also a source of allergy-causing proteins. When the substance carrying the proteins dries, the proteins can then float into the air. Cats may be more likely than dogs to cause allergic reactions because they lick themselves more and may be held more and spend more time in the house, close to humans.

Some rodents, such as guinea pigs and gerbils, have become increasingly popular as household pets. They, too, can cause allergic reactions in some people, as can mice and rats. Urine is the major source of allergens from these animals.

Allergies to animals can take two years or more to develop and may not subside until six months or more after ending contact with the animal. Carpet and furniture are a reservoir for pet allergens, and the allergens can remain in them for four to six weeks. In addition, these allergens can stay in household air for months after the animal has been removed. Therefore, it is wise for people with an animal allergy to check with the landlord or previous owner to find out if furry pets had lived previously on the premises.

Doctors use skin tests to determine whether a patient has IgE antibodies in the skin that react to a specific allergen. The doctor will use diluted extracts from allergens such as dust mites, pollens, or molds commonly found in the local area. The extract of each kind of allergen is injected under the patient's skin or is applied to a tiny scratch or puncture made on the patient's arm or back.

Skin tests are one way of measuring the level of IgE antibody in a patient. With a positive reaction, a small, raised, reddened area (called a wheal) with a surrounding flush (called a flare) will appear at the test site. The size of the wheal can give the physician an important diagnostic clue, but a positive reaction does not prove that a particular pollen is the cause of a patient's symptoms. Although such a reaction indicates that IgE antibody to a specific allergen is present in the skin, respiratory symptoms do not necessarily result.

Complete avoidance of allergenic pollen or mold means moving to a place where the offending substance does not grow and where it is not present in the air. But even this extreme solution may offer only temporary relief since a person who is sensitive to a specific pollen or mold may subsequently develop allergies to new allergens after repeated exposure. For example, people allergic to ragweed may leave their ragweed-ridden communities and relocate to areas where ragweed does not grow, only to develop allergies to other weeds or even to grasses or trees in their new surroundings. Because relocating is not a reliable solution, allergy specialists do not encourage this approach.

There are other ways to evade the offending pollen: remaining indoors in the morning, for example, when the outdoor pollen levels are highest. Sunny, windy days can be especially troublesome. If individuals with pollen allergy must work outdoors, they can wear face masks designed to filter pollen out of the air and keep it from reaching their nasal passages. As another approach, some people take their vacations at the height of the expected pollinating period and choose a location where such exposure would be minimal. The seashore, for example, may be an effective retreat for many with pollen allergies.

Mold allergens can be difficult to avoid, but some steps can be taken to at least reduce exposure to them. First, the allergy sufferer should avoid those hot spots mentioned earlier where molds tend to be concentrated. The lawn should be mowed and leaves should be raked up, but someone other than the allergic person should do these chores. If such work cannot be delegated, wearing a tightly fitting dust mask can greatly reduce exposure and resulting symptoms. Travel in the country, especially on dry, windy days or while crops are being harvested, should be avoided as should walks through tall vegetation. A summer cabin closed up all winter is probably full of molds and should be aired out and cleaned before a mold-sensitive person stays there.

Around the home, a dehumidifier will help dry out the basement, but the water extracted from the air must be removed frequently to prevent mold growth in the machine.

Those with dust mite allergy should pay careful attention to dust-proofing their bedrooms. The worst things to have in the bedroom are wall-to-wall carpets, venetian blinds, down-filled blankets, feather pillows, heating vents with forced hot air, dogs, cats, and closets full of clothing. Shades are preferred over venetian blinds because they do not trap dust. Curtains can be used if they are washed periodically in hot water to kill the dust mites. Most important, bedding should be encased in a zippered, plastic, airtight, and dust-proof cover.

Although shag carpets are the worst type for the dust mite-sensitive person, all carpets trap dust and make dust control impossible. In addition, vacuuming can contribute to the amount of dust, unless the vacuum is equipped with a special high-efficiency particulate air (HEPA) filter. Wall-to-wall carpets should be replaced with washable throw rugs over hardwood, tile, or linoleum floors. Rugs on concrete floors encourage dust mite growth and should be avoided.

Reducing the amount of dust mites in a home may require new cleaning techniques as well as some changes in furnishings to eliminate dust collectors. Water is often the secret to effective dust removal. Washable items should be washed often using water hotter then 130 (degrees) Fahrenheit. Lower temperatures will not kill dust mites. If the water temperature must be set at a lower value, items can be washed at a commercial establishment that uses high wash temperatures. Dusting with a damp cloth or oiled mop should be done frequently.

The best way for a person allergic to pets, especially cats, to avoid allergic reactions is to find another home for the animal. There are, however, some suggestions that help lower the levels of cat allergens in the air.

• Bathe the cat weekly and brush it more frequently (ideally, this should be done by someone other than the allergic person).
• Remove carpets and soft furnishings.
• Use a vacuum cleaner with a high-efficiency filter.
• Use a room air cleaner (see section below).
• Wear a face mask while house and cat cleaning and keeping the cat out of the bedroom are other methods that allow many people to live more happily with their pets.

Irritants such as chemicals can worsen airborne allergy symptoms and should be avoided as much as possible. For example, during periods of high pollen levels, people with pollen allergy should try to avoid unnecessary exposure to irritants such as insect sprays, tobacco smoke, air pollution, and fresh tar or paint.

When possible, an allergic person should use air conditioners inside the home or in a car to help prevent pollen and mold allergens from entering. Various types of air-filtering devices made with fiberglass or electrically charged plates may help reduce allergens produced in the home. These can be added to the heating and cooling systems. In addition, portable devices that can be used in individual rooms are especially helpful in reducing animal allergens.

An allergy specialist can suggest which kind of filter is best for the home of a particular patient. Before buying a filtering device, the patient should rent one and use it in a closed room (the bedroom, for instance) for a month or two to see whether allergy symptoms diminish. The airflow should be sufficient to exchange the air in the room five or six times per hour; therefore, the size and efficiency of the filtering device should be determined in part by the size of the room.

Persons with allergies should be wary of exaggerated claims for appliances that cannot really clean the air. Very small air cleaners cannot remove dust and pollen--and no air purifier can prevent viral or bacterial diseases such as influenza, pneumonia, or tuberculosis. Buyers of electrostatic precipitators should compare the machine's ozone output with Federal standards. Ozone can irritate the nose and airways of persons with allergies, especially those with asthma, and can increase the allergy symptoms. Other kinds of air filters such as HEPA filters do not release ozone into the air. HEPA filters, however, require adequate air flow to force air through them.

For people who find they cannot adequately avoid airborne allergens, the symptoms often can be controlled with medications. Effective medications that can be prescribed by a physician include antihistamines and topical nasal steroids--either of which can be used alone or in combination. Many effective antihistamines and decongestants also are available without a prescription.

Antihistamines. As the name indicates, an antihistamine counters the effects of histamine, which is released by the mast cells in the body's tissues and contributes to allergy symptoms. For many years, antihistamines have proven useful in relieving sneezing and itching in the nose, throat, and eyes, and in reducing nasal swelling and drainage.

Many people who take antihistamines experience some distressing side effects: drowsiness and loss of alertness and coordination. In children, such reactions can be misinterpreted as behavior problems. During the last few years, however, antihistamines that cause fewer of these side effects have become available by prescription. These non-sedating antihistamines are as effective as other antihistamines in preventing histamine-induced symptoms, but do so without causing sleepiness. Some of these non-sedating antihistamines, however, can have serious side effects, particularly if they are taken with certain other drugs. A patient should always let the doctor know what other medications he/she is taking.

Topical nasal steroids. This medication should not be confused with anabolic steroids, which are sometimes used by athletes to enlarge muscle mass and can have serious side effects. Topical nasal steroids are anti-inflammatory drugs that stop the allergic reaction. In addition to other beneficial actions, they reduce the number of mast cells in the nose and reduce mucus secretion and nasal swelling. The combination of antihistamines and nasal steroids is a very effective way to treat allergic rhinitis, especially in people with moderate or severe allergic rhinitis. Although topical nasal steroids can have side effects, they are safe when used at recommended doses. Some of the newer agents are even safer than older ones.

Cromolyn sodium. Cromolyn sodium for allergic rhinitis is a nasal spray that in some people helps to prevent allergic reactions from starting. When administered as a nasal spray, it can safely inhibit the release of chemicals like histamine from the mast cell. It has few side effects when used as directed, and significantly helps some patients with allergies.

Decongestants. Sometimes re-establishing drainage of the nasal passages will help to relieve symptoms such as congestion, swelling, excess secretions, and discomfort in the sinus areas that can be caused by nasal allergies. (These sinus areas are hollow air spaces located within the bones of the skull surrounding the nose.) The doctor may recommend using oral or nasal decongestants to reduce congestion along with an antihistamine to control allerigic symptoms. Over-the-counter and prescription decongestant nose drops and sprays, however, should not be used for more than a few days. When used for longer periods, these drugs can lead to even more congestion and swelling of the nasal passages.

The National Institute of Allergy and Infectious Diseases (NIAID) conducts and supports research on allergies focused on understanding what happens to the body during the allergic process--the sequence of events leading to the allergic response and the factors responsible for allergic diseases. This understanding will lead to better methods of diagnosing, preventing, and treating allergies.

NIAID supports a network of Asthma, Allergic and Immunologic Diseases Cooperative Research Centers throughout the United States. The centers encourage close coordination among scientists studying basic and clinical immunology, genetics, biochemistry, pharmacology, and environmental science. This interdisciplinary approach helps move research knowledge as quickly as possible from research scientists to physicians and their allergy patients.

Educating patients and health care workers is an important tool in controlling allergic diseases. All of these research centers conduct and evaluate educational programs focused on methods to control allergic diseases.

Researchers participating in NIAID's National Cooperative Inner-City Asthma Study are examining ways to prevent asthma in minority children in inner-city environments. Asthma, a major cause of illness and hospitalizations among these children, is provoked by a number of possible factors, including allergies to airborne substances.

Although several factors provoke allergic responses, scientists know that heredity is a major influence on who will develop an allergy. Therefore, researchers are trying to identify and describe the genes that make a person susceptible to allergic diseases.

Some studies are aimed at seeking better ways to diagnose and treat people with allergic diseases and to better understand the factors that regulate IgE production in order to reduce the allergic response in patients. Several research institutions are focusing on ways to influence the cells that participate in the allergic response.

Because researchers are becoming increasingly aware of the role of environmental factors in allergies, they are evaluating ways to control environmental exposures to allergens and pollutants to prevent allergic disease.

These studies offer the promise of improving treatment and control of allergic diseases and the hope that one day allergic diseases will be preventable as well.