An allergic reaction - Something in the air (by Ahmedabad Medical Guide)

An allergic reaction - Something in the air

Sneezing is not always the symptom of a cold. Sometimes, it is an allergic reaction to something in the air. Experts estimate that 35 million Americans suffer from upper respiratory symptoms that are allergic reactions to airborne pollen. 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.

What is an allergy?

An allergy is a specific immunologic reaction to a normally harmless substance, one that does not bother most people. People who have allergies often are sensitive to more than one substance. Types of allergens that cause allergic reactions include pollens, dust particles, mold spores, food, latex rubber, insect venom, or medicines.

Why are some people allergic to these substances while others are not?

Scientists think that people inherit a tendency to be allergic, meaning an increased likelihood of being allergic to one or more allergens, although they probably do not have an inherited tendency to be allergic to any specific allergens. Children are much more likely to develop allergies if their parents have allergies, even if only one parent is allergic. Exposure to allergens at certain times when the body's defenses are lowered or weakened, such as after a viral infection or during pregnancy, seems to contribute to the development of allergies.

What is an allergic reaction?

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.

Symptoms of Allergies to Airborne Substances

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.

Dust Mite Allergy

Dust mite allergy is an allergy to a microscopic organism that lives in the dust that is found in all dwellings and workplaces. Dust mites are perhaps the most common cause of perennial allergic rhinitis. Dust mite allergy usually produces symptoms similar to pollen allergy and also can produce symptoms of asthma.

What is house dust?

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.

Animal Allergy

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.

Chemical Sensitivity

Some people report that they react to chemicals in their environment and that these allergy-like reactions appear to result from exposure to a wide variety of synthetic and natural substances, such as those found in paints, carpeting, plastics, perfumes, cigarette smoke, and plants. Although the symptoms may resemble some of the manifestations of allergies, sensitivity to chemicals does not represent a true allergic reaction involving IgE and the release of histamine or other chemicals.

Diagnosing Allergic Diseases

People with allergy symptoms, such as the runny nose of allergic rhinitis, may at first suspect they have a cold--but the "cold" lingers on. It is important to see a doctor about any respiratory illness that lasts longer than a week or two. When it appears that the symptoms are caused by an allergy, the patient should see a physician who understands the diagnosis and treatment of allergies. If the patient's medical history indicates that the symptoms recur at the same time each year, the physician will work under the theory that a seasonal allergen (like pollen) is involved. Properly trained specialists recognize the patterns of potential allergens common during local seasons and the association between these patterns and symptoms. The medical history suggests which allergens are the likely culprits. The doctor also will examine the mucous membranes, which often appear swollen and pale or bluish in persons with allergic conditions.

Skin Tests

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.

Blood Tests

Although skin testing is the most sensitive and least costly way to identify allergies in patients, some patients such as those with widespread skin conditions like eczema should not be tested using that method. There are other diagnostic tests that use a blood sample from the patient to detect levels of IgE antibody to a particular allergen. One such blood test is called the RAST (radioallergosorbent test), which can be performed when eczema is present or if a patient has taken medications that interfere with skin testing.

Treating People with Allergic Diseases

Doctors use three general approaches to helping people with allergies: advise them on ways to avoid the allergen as much as possible, prescribe medication to relieve symptoms, and give a series of allergy shots. Although there is no cure for allergies, one of these strategies or a combination of them can provide varying degrees of relief from allergy symptoms.

Air conditioners and filters

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.

Medications

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.

Immunotherapy

Immunotherapy, or a series of allergy shots, is the only available treatment that has a chance of reducing the allergy symptoms over a longer period of time. Patients receive subcutaneous (under the skin) injections of increasing concentrations of the allergen(s) to which they are sensitive. These injections reduce the amount of IgE antibodies in the blood and cause the body to make a protective antibody called IgG. Many patients with allergic rhinitis will have a significant reduction in their hay fever symptoms and in their need for medication within 12 months of starting immunotherapy. Patients who benefit from immunotherapy may continue it for three years and then consider stopping. Although many patients are able to stop the injections with good, long-term results, some do get worse after immunotherapy is stopped. As better allergens for immunotherapy are produced, this technique will become an even more effective treatment.

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