Allergies

Definition
Allergies are abnormal reactions of the immune system that occur in response to otherwise harmless substances.

Description
Allergies are among the most common of medical disorders. It is estimated that 60 million Americans, or more than one in every five people, suffer from some form of allergy, with similar proportions throughout much of the rest of the world. Allergy is the single largest reason for school absence and is a major source of lost productivity in the workplace.
An allergy is a type of immune reaction. Normally, the immune system responds to foreign microorganisms or particles by producing specific proteins called antibodies. These antibodies are capable of binding to identifying molecules, or antigens, on the foreign particle. This reaction between antibody and antigen sets off a series of chemical reactions designed to protect the body from infection. Sometimes, this same series of reactions is triggered by harmless, everyday substances such as pollen, dust, and animal danders. When this occurs, an allergy develops against the offending substance (an allergen.)
Mast cells, one of the major players in allergic reactions, capture and display a particular type of antibody, called immunoglobulin type E (IgE) that binds to allergens. Inside mast cells are small chemical-filled packets called granules. Granules contain a variety of potent chemicals, including histamine.
Immunologists separate allergic reactions into two main types: immediate hypersensitivity reactions, which are predominantly mast cell-mediated and occur within minutes of contact with allergen; and delayed hypersensitivity reactions, mediated by T cells (a type of white blood cells) and occurring hours to days after exposure.
Inhaled or ingested allergens usually cause immediate hypersensitivity reactions. Allergens bind to IgE antibodies on the surface of mast cells, which spill the contents of their granules out onto neighboring cells, including blood vessels and nerve cells. Histamine binds to the surfaces of these other cells through special proteins called histamine receptors. Interaction of histamine with receptors on blood vessels causes increased leakiness, leading to the fluid collection, swelling and increased redness. Histamine also stimulates pain receptors, making tissue more sensitive and irritable. Symptoms last from one to several hours following contact.
In the upper airways and eyes, immediate hypersensitivity reactions cause the runny nose and itchy, bloodshot eyes typical of allergic rhinitis. In the gastrointestinal tract, these reactions lead to swelling and irritation of the intestinal lining, which causes the cramping and diarrhea typical of food allergy. Allergens that enter the circulation may cause hives, angioedema, anaphylaxis, or atopic dermatitis.
Allergens on the skin usually cause delayed hypersensitivity reaction. Roving T cells contact the allergen, setting in motion a more prolonged immune response. This type of allergic response may develop over several days following contact with the allergen, and symptoms may persist for a week or more.

Causes and symptoms
Allergens enter the body through four main routes: the airways, the skin, the gastrointestinal tract, and the circulatory system.
Airborne allergens cause the sneezing, runny nose, and itchy, bloodshot eyes of hay fever (allergic rhinitis). Airborne allergens can also affect the lining of the lungs, causing asthma, or conjunctivitis (pink eye). Exposure to cockroach allergens has been associated with the development of asthma. Airborne allergens from household pets are another common source of environmental exposure.
Allergens in food can cause itching and swelling of the lips and throat, cramps, and diarrhea. When absorbed into the bloodstream, they may cause hives (urticaria) or more severe reactions involving recurrent, non-inflammatory swelling of the skin, mucous membranes, organs, and brain (angioedema). Some food allergens may cause anaphylaxis, a potentially life-threatening condition marked by tissue swelling, airway constriction, and drop in blood pressure. Allergies to foods such as cow's milk, eggs, nuts, fish, and legumes (peanuts and soybeans) are common. Allergies to fruits and vegetables may also occur.
In contact with the skin, allergens can cause reddening, itching, and blistering, called contact dermatitis. Skin reactions can also occur from allergens introduced through the airways or gastrointestinal tract. This type of reaction is known as atopic dermatitis. Dermatitis may arise from an allergic response (such as from poison ivy), or exposure to an irritant causing nonimmune damage to skin cells (such as soap, cold, and chemical agents).
Injection of allergens, from insect bites and stings or drug administration, can introduce allergens directly into the circulation, where they may cause system-wide responses (including anaphylaxis), as well as the local ones of swelling and irritation at the injection site.
People with allergies are not equally sensitive to all allergens. Some may have severe allergic rhinitis but no food allergies, for instance, or be extremely sensitive to nuts but not to any other food. Allergies may get worse over time. For example, childhood ragweed allergy may progress to year-round dust and pollen allergy. On the other hand, a person may lose allergic sensitivity. Infant or childhood atopic dermatitis disappears in almost all people. More commonly, what seems to be loss of sensitivity is instead a reduced exposure to allergens or an increased tolerance for the same level of symptoms.
While allergy to specific allergens is not inherited, the likelihood of developing some type of allergy seems to be, at least for many people. If neither parent has allergies, the chances of a child developing allergy is approximately 10-20%; if one parent has allergies, it is 30-50%; and if both have allergies, it is 40-75%. One source of this genetic predisposition is in the ability to produce higher levels of IgE in response to allergens. Those who produce more IgE will develop a stronger allergic sensitivity.

COMMON ALLERGENS
The most common airborne allergens are the following:
plant pollens
animal fur and dander
body parts from house mites (microscopic creatures found in all houses)
house dust
mold spores
cigarette smoke
solvents
cleaners
Common food allergens include the following:
The following types of drugs commonly cause allergic reactions:
penicillin or other antibiotics
flu vaccines
tetanus toxoid vaccine
gamma globulin
Common causes of contact dermatitis include the following:
poison ivy, oak, and sumac
nickel or nickel alloys
latex
Insects and other arthropods whose bites or stings typically cause allergy include the following:
bees, wasps, and hornets
mosquitoes
fleas
scabies

Symptoms depend on the specific type of allergic reaction. Allergic rhinitis is characterized by an itchy, runny nose, often with a scratchy or irritated throat due to post-nasal drip. Inflammation of the thin membrane covering the eye (allergic conjunctivitis) causes redness, irritation, and increased tearing in the eyes. Asthma causes wheezing, coughing, and shortness of breath. Symptoms of food allergies depend on the tissues most sensitive to the allergen and whether the allergen was spread systemically by the circulatory system. Gastrointestinal symptoms may include swelling and tingling in the lips, tongue, palate or throat; nausea; cramping; diarrhea; and gas. Contact dermatitis is marked by reddened, itchy, weepy skin blisters, and an eczema that is slow to heal. It sometimes has a characteritic man-made pattern, such as a glove allergy with clear demarkation on the hands, wrist, and arms where the gloves are worn, or on the earlobes by wearing earrings.
Whole body or systemic reactions may occur from any type of allergen, but are more common following ingestion or injection of an allergen. Skin reactions include the raised, reddened, and itchy patches called hives that characteristically blanch with pressure and resolve within twenty-four hours. A deeper and more extensive skin reaction, involving more extensive fluid collection and pain, is called angioedema. This usually occurs on the extremities, fingers, toes, and parts of the head, neck, and face. Anaphylaxis is marked by airway constriction, blood pressure drop, widespread tissue swelling, heart rhythm abnormalities, and in some cases, loss of consciousness. Other syptoms may include, dizziness, weakness, seizures, coughing, flushing, or cramping. The symptoms may begin within five minutes after exposure to the allergen up to one hour or more later. Mast cells in the tissues and basophils in the blood release mediators that give rise to the clinical symptoms of this IgE-mediated hypersensitivity reaction. Commonly, this is associated with allergies to medications, foods, and insect venoms. In some individuals, anaphylaxis can occur with exercise, plasma exchange, hemodialysis, reaction to insulin, contrast media used in certain types of medical tests, and rarely during the administration of local anesthetics.

Diagnosis
Allergies can often be diagnosed by a careful medical history, matching the onset of symptoms to the exposure to possible allergens. Allergy is suspected if the symptoms presented are characteristic of an allergic reaction and this occurs repeatedly upon exposure to the suspected allergen. Allergy tests can be used to identify potential allergens, but these must be supported by eveidence of allergic responses in the patient's clinical history.

Skin tests
Skin tests are performed by administering a tiny dose of the suspected allergen by pricking, scratching, puncturing or injecting the skin. The allergen is applied to the skin as an auqeous extract, usually on the back, forearms, or top of the thighs. Once in the skin, the allergen may produce a classic immune wheal and flare response (a skin lesion with a raised, white, compressible area surrounded by a red flare). The tests usually begin with prick tests or patch tests that expose the skin to small amounts of allergen to observe the response. A positive reaction will occur on the skin even if the allergen is at levels normally encountered in food or in the airways. Reactions are usually evaluated approximately fifteen minutes after exposure. Intradermal skin tests involved injection of the allergen into the dermis of the skin. These tests are more sensitive and are used for allergies associated with risk of death, such as allergies to antibiotics.

Allergen-Specific IgE Measurement
Tests that measure allergen-specific IgE antibodies generally follow a basic method. The allergen is bound to a solid support, either in the form of a cellulose sponge, microtiter plate, or paper disk. The patient's serum is prepared from a blood sample and is incubated with the solid phase. If allergen specific IgE antibodies are present, they will bind to the solid phase and be retained there when the rest of the serum is washed away. Next, an labeled antibody against the IgE is added and will bind to any IgE on the solid phase. The excess is washed away and the levels of IgE are determined. The commonly used RAST test (radio allergo sorbent test) employed radio-labeled Anti-IgE antibodies. Updated methods now incorporate the use of enyzme-labeled antibodies in ELISA assays (enzyme-linked immunosorbent assays).

Total Serum IgE
The total level of IgE in the serum is commonly measured with a two-site immunometric assay. Some research indicates that there is a higher level of total serum IgE in allergic as compared to non-allergic people. However, this may not always be the case as there is considerable overlap between the two groups. This test is useful for the diagnosis of allergic fungal sinusitis and bronchopulmonary aspergillosis. Other conditions that are not allergic in nature may give rise to higher IgE levels such as smoking, AIDS, infection with parasites, and IgE myeloma.

Provocation tests
These tests involve the administration of allergen to elicit an immune response. Provocation tests, most commonly done with airborne allergens, present the allergen directly through the route normally involved. Delayed allergic contact dermatitis diagnosis involves similar methods by application of a skin patch with allergen to induce an allergic skin reaction. Food allergen provocation tests require abstinence from the suspect allergen for two weeks or more, followed by ingestion of a measured amount of the test substance administered as an opaque capsule along with a placebo control. Provocation tests are not used if anaphylaxis is is a concern due to the patient's medical history.

Future diagnostic methods
Attempts have been made for direct measurement of immune mediators such as histamine, eosinophil cationic protein (ECP), and mast cell tryptase. Another, somewhat controversial,test is electrodermal testing or electro-acupuncture allergy testing. This test has been used in Europe and is under investigation in the United States, though not approved by the Food and Drug Administration. An electric potential is applied to the skin, the allergen presented, and the electrical resistance observed for changes. This method has not been verified.

Treatment
Avoiding allergens is the first line of defense to reduce the possibility of an allergic attack. It is helpful to avoid environmental irritants such as tobacco smoke, perfumes, household cleaning agents, paints, glues, air fresheners, and potpourri. Nitrogen dioxide from poorly vented gas stoves, woodburning stoves, and artificial fireplaces has also been linked to poor asthma control. Dust mite control is particularly important in the bedroom areas by use of allergen-impermeable covers on mattress and pillows, frequent washing of bedding in hot water, and removal of items that collect dust such as stuffed toys. Mold growth may be reduced by lowering indoor humidity, repair of house foundations to reduce indoor leaks and seepage, and installing exhaust systems to ventilate areas where steam is generated such as the bathroom or kitchen. Allergic individuals should avoid pet allergens such as saliva, body excretions, pelts, urine, or feces. For those who insist on keeping a pet, restriction of the animal's activity to certain areas of the home may be beneficial.
Complete environmental control is often difficult to accomplish, hence therapuetic interventions may become necessary. A large number of prescription and over-the-counter drugs are available for treatment of immediate hypersensitivity reactions. Most of these work by decreasing the ability of histamine to provoke symptoms. Other drugs counteract the effects of histamine by stimulating other systems or reducing immune responses in general.

Antihistamines
Antihistamines block the histamine receptors on nasal tissue, decreasing the effect of histamine released by mast cells. They may be used after symptoms appear, though they may be even more effective when used preventively, before symptoms appear. Antihistamines help reduce sneezing, itching, and rhinorrhea. A wide variety of antihistamines are available.
Older, first generation antihistamines often produce drowsiness as a major side effect, as well as dry mouth, tachycardia, blurred vision, constipation, and lower the threshold for seizures. These medications also have similar effects to a alcohol and care should be taken when operating motor vehicles, as individuals may not be aware that they are impaired. Such antihistamines include the following:
diphenhydramine (Benadryl and generics)
chlorpheniramine (Chlor-trimeton and generics)
brompheniramine (Dimetane and generics)
clemastine (Tavist and generics)
Newer antihistamines that do not cause drowsiness or pass the blood-brain barrier are available by prescription and include the following:
loratidine (Claritin)
cetirizine (Zyrtec)
fexofenadine (Allegra)
Desloratadine (Clarinex) was approved in 2004 in syrup form for children two years and older for seasonal allergies and for hives of unknown cause in children as young as six months. It is the only nonsedating antihistamine approved as of 2004 for children as young as six months.
Hismanal has the potential to cause serious heart arrhythmias when taken with the antibiotic erythromycin, the antifungal drugs ketoconazole and itraconazole, or the antimalarial drug quinine. Taking more than the recommended dose of Hismanal can also cause arrhythimas. Seldane (terfenadine), the original non-drowsy antihistamine, was voluntarily withdrawn from the market by its manufacturers in early 1998 because of this potential and because of the availability of an equally effective, safer alternative drug, fexofenadine.

Decongestants
Decongestants constrict blood vessels to the mucosa to counteract the effects of histamine. This decreases the amount of blood in the nasopahryngeal and sinus mucosa and reduces swelling. Nasal sprays are available that can be applied directly to the nasal lining and oral systematic preparations are available. Decongestants are stimulants and may cause increased heart rate and blood pressure, headaches, insomnia, agitation, and difficulty emptying the bladder. Use of topical decongestants for longer than several days can cause loss of effectiveness and rebound congestion, in which nasal passages become more severely swollen than before treatment.

Topical corticosteroids
Topical corticosteroids reduce mucous membrane inflammation by decreasing the amount of fluid moved from the vascular spaces into the tissues. These medications reduce the recruitment of inflammatory cells as well as the synthesis of cytokines. They are available by prescription. Allergies tend to become worse as the season progresses because the immune system becomes sensitized to particular antigens and can produce a faster, stronger response. Topical corticosteroids are especially effective at reducing this seasonal sensitization because they work more slowly and last longer than most other medication types. As a result, they are best started before allergy season begins. Side effects are usually mild, but may include headaches, nosebleeds, and unpleasant taste sensations.

Bronchodilators or metered-dose inhalers (MDI)
Because allergic reactions involving the lungs cause the airways or bronchial tubes to narrow, as in asthma, bronchodilators, which cause the smooth muscle lining the airways to open or dilate, can be very effective. When inhalers are used, it is important that the patient be educated in the proper use of these medications. The inhaler should be shaken, and the patient should breathe out to expel air from the lungs. The inhaler should be placed at least two fingerbreadths in front of the mouth. The medication should be aimed at the back of the throat, and the inhaler activated while breathing in quite slowly 3-4 seconds. The breath should be held for at least ten seconds, and then expelled. At least thirty to sixty seconds should pass before the inhaler is used again. Care should be taken to properly wash out the mouth and brush the teeth following use, as residual medication remains in this area with only a small amount actually reaching the lungs. Some bronchodilators used to treat acute asthma attacks include adrenaline, albuterol, Maxair, Proventil, or other "adrenoceptor stimulants," most often administered as aerosols. Successfully managing asthma and allergies can reduce the use of inhalers. This is done through good communication between the physician and patient, self-management with written action plans, avoiding allergy triggers, and through the use of preventive medications such as montelukast.

Anticholinergics
Ipratropium bromide (atrovent) and atropine sulfate are achticholinergic drugs used for the treatment of asthma. Ipratropium is used for treating asthmatics in emergency situations with a nebulizer.

Nonsteroidal drugs

MAST CELL STABILIZERS
Cromolyn sodium prevents the release of mast cell granules, thereby preventing the release of histamine and other chemicals contained in them. It acts as a preventive treatment if it is begun several weeks before the onset of the allergy season. It can also be used for year round allergy prevention. Cromolyn sodium is available as a nasal spray for allergic rhinitis and in aerosol (a suspension of particles in gas) form for asthma.

LEUKOTRIENE MODIFIERS
These medications are useful for individuals with aspirin sensitivity, sinusitis, polposis, urticaria. Examples include zafirlukast (Accolate), montelukast (Singulair), and zileuton (Zyflo). When zileuton is used, care must be taken to measure liver enzymes.

Immunotherapy
In this form of therapy, allergen is injected into the skin in increasing doses over a specific period of time. This may be helpful for patients who do not respond to medications or avoidance of allergens in the environment. This type of therapy may reduce the need for medications. A 2004 study recommended that children who have severe reactions to insect sting receive immunotherapy to protect them against future stings.

Treatment of contact dermatitis
An individual suffering from contact dermatitis should initially take steps to avoid possible sources of exposure to the offending agent. Calamine lotion applied to affected skin can reduce irritation somewhat, as can cold water compresses. Side effects of topical agents may include over-drying of the skin. In the case of acute contact dermatitis, short-term oral corticosteroid therapy may be appropriate. Moderately strong coricosteroids can also be applied as a wrap for twenty-four hours. Health care workers are especially at risk for hand eruptions due to glove use.

Treatment of anaphylaxis
The emergency condition of anaphylaxis is treated with injection of adrenaline, also known as epinephrine. People who are prone to anaphylaxis because of food or insect allergies often carry an "Epi-pen" containing adrenaline in a hypodermic needle. Other medications may be given to aid the action of the epi-pen. Prompt injection can prevent a more serious reaction from developing. Particular care should be taken to assess the affected individual's airway status, and he or she should be placed in a recumbent pose and vital signs determined. If a reaction resulted from insect sting or an injection, a tourniquet may need to be placed proximal to the area where the agent penetrated the skin. This should then be released at intervals of ten minutes at a time, for one to two minutes duration. If the individual does not respond to such interventions, then emergency treatment is appropriate.

Alternative treatment
Any alternative treatment for allergies begins with finding the cause and then helping the patient to avoid or eliminate the allergen, although this is not always possible. As with any alternative therapy, a physician should be consulted before initiating a new form of treatment. Education on the use of alternative agents is critical, as they are still "drugs" even though they are derived from natural sources. Various categories of alternative remedies may be helpful in allergy treatment, including:
antihistamines: vitamin C and the bioflavonoid hesperidin act as natural anithistamines.
decongestants: vitamin C, the homeopathic remedies Ferrum phosphoricum and Kali muriaticum (used alternately), and the dietary supplement N-acetylcysteine are believed to have decongestant effects.
mast cell stabilizers: the bioflavonoids quercetin and hesperidin may help stabilize mast cells.
immunotherapy: the herbs echinacea (Echinacea spp.) and astragalus or milk-vetch root (Astragalus membranaceus) may possibly help to strengthen the immune system.
bronchodilators: the herbal remedies ephedra (Ephedra sinica, also known as ma huang in traditional Chinese medicine), khellin (Ammi visnaga) and cramp bark (Viburnum opulus) are believed to help open the airways.

Treatment of contact dermatitis
A variety of herbal remedies, either applied topically or taken internally, may possibly assist in the treatment of contact dermatitis. A poultice (crushed herbs applied directly to the affected area) made of jewelweed (Impatiens spp.) or chickweed (Stellaria media) may soothe the skin. A cream or wash containing calendula (Calendula officinalis), a natural antiseptic and anti-inflammatory agent, may help heal the rash when applied topically. Homeopathic treatment may include such remedies as Rhus toxicodendron, Apis mellifica, or Anacardium taken internally. A qualified homeopathic practitioner should be consulted to match the symptoms with the correct remedy. Care should be taken with any agent taken internally.

Prognosis
Allergies can improve over time, although they often worsen. While anaphylaxis and severe asthma are life-threatening, other allergic reactions are not. Learning to recognize and avoid allergy-provoking situations allows most people with allergies to lead normal lives.

Prevention
Avoiding allergens is the best means of limiting allergic reactions. For food allergies, there is no effective treatment except avoidance. By determining the allergens that are causing reactions, most people can learn to avoid allergic reactions from food, drugs, and contact allergens such as poison ivy or latex. The government will help now, since passing the Food Allergen Labeling and Consumer Protection Act in 2004. Beginning January 1, 2006, food manufacturers will be required to clearly state if a product contains any of the eight major food allergens that are responsible for more than 90% of allergic reactions to foods. These are milk, eggs, peanuts, tree nuts, fish, shellfish, wheat, and soy.
Airborne allergens are more difficult to avoid, although keeping dust and animal dander from collecting in the house may limit exposure. Cromolyn sodium can prevent mast cell degranulation, thereby limiting the allergic response.
Immunotherapy, also known as desensitization or allergy shots, alters the balance of antibody types in the body, thereby reducing the ability of IgE to cause allergic reactions. Immunotherapy is preceded by allergy testing to determine the precise allergens responsible. Injections involve very small but gradually increasing amounts 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 all in about one in five patients. Individuals receiving all shots will be monitored closely following each shot because of the small risk of anaphylaxis, a condition that can result in difficulty breathing and a sharp drop in blood pressure.
Other drugs, such as leukotriene modifiers, are used to prevent asthma attacks and in the long-term management of allergies and asthma.

Key Terms
Allergen
A substance that provokes an allergic response.
Allergic rhinitis
Inflammation of the mucous membranes of the nose and eyes in response to an allergen.
Anaphylaxis
Increased sensitivity caused by previous exposure to an allergen that can result in blood vessel dilation and smooth muscle contraction. Anaphylaxis can result in sharp blood pressure drops and difficulty breathing.
Angioedema
Severe non-inflammatory swelling of the skin, organs, and brain that can also be accompanied by fever and muscle pain.
Antibody
A specific protein produced by the immune system in response to a specific foreign protein or particle called an antigen.
Antigen
A foreign protein to which the body reacts by making antibodies.
Asthma
A lung condition in which the airways become narrow due to smooth muscle contraction, causing wheezing, coughing, and shortness of breath.
Atopic dermatitis
Infection of the skin as a result of exposure to airborne or food allergens.
Conjunctivitis
Inflammation of the thin lining of the eye called the conjunctiva.
Contact dermatitis
Inflammation of the skin as a result of contact with a substance.
Delayed hypersensitivity reactions
Allergic reactions mediated by T cells that occur hours to days after exposure.
Granules
Small packets of reactive chemicals stored within cells.
Histamine
A chemical released by mast cells that activates pain receptors and causes cells to become leaky.
Immune hypersensitivity reaction
Allergic reactions that are mediated by mast cells and occur within minutes of allergen contact.
Mast cells
A type of immune system cell that is found in the lining of the nasal passages and eyelids, displays a type of antibody called immunoglobulin type E (IgE) on its cell surface, and participates in the allergic response by releasing histamine from intracellular granules.
T cells
Immune system cells or more specifically, white blood cells, that stimulate cells to create and release antibodies.

For Your Information
Resources/Books
Hans-Uwe, Simon, editor. CRC Desk Reference for Allergy and Asthma. Boca Raton: CRC Press, 2000.
Kemp, Stephen F., and Richard Lockey, editors. Diagnostic Testing of Allergic Disease. New York: Marcel Dekker, Inc., 2000.
Lieberman, Phil, and Johh Anderson, editors. Allergic Diseases: Diagnosis and Treatment. 2nd ed. Totowa: Humana Press, Inc., 2000.
Periodicals
"Children With Serious Insect-sting Allergies Should Get Shots." Drug Week (September 3, 2004): 19.
"FDA Approves Clarinex Syrup for Allergies and Hives in Children." Biotech Week (September 29, 2004): 617.
"President Bush Signs Bill that Will Benefit Millions With Food Allergies." Immunotherapy Weekly (September 1, 2004): 50.
"What's New in: Asthma and Allergic Rhinitis." Pulse (September 20, 2004): 50.

Gale Encyclopedia of Medicine, Published December, 2002 by the Gale Group The Essay Author is Richard Robinson.

More Information
Like clockwork, the sneezing and sniffling hits you every spring. Or maybe it's every fall. But chances are it's not a cold. You have allergies. So what are allergies? They are exaggerated reactions of the immune system to substances that in most people don't cause problems. Symptoms may be caused by different exposures such as a certain chemical affecting the skin, dust or pollen particles affecting the respiratory system, or particular foods affecting the stomach and intestines.
As many as 40 million to 50 million Americans may have allergies, according to the American Academy of Allergy, Asthma and Immunology. Allergies affect more than 20 percent of the U.S. population and allergic diseases are the sixth leading cause of chronic disease in the United States. And at least 35 million have seasonal allergic rhinitis, also known as hay fever.
Common allergens are pollen, molds, dust mites, animal dander, certain foods and medications, and latex.
Allergic rhinitis (AR), or hay fever, happens when an allergen is inhaled through the mouth or nose. Sometimes symptoms are year-round, meaning it could be triggered by dust mites, cockroaches, animal dander, pollen or mold spores. AR is the most common allergy and is considered a risk in developing asthma. Nearly 80 percent of people with asthma also have AR. Sinusitis is a common disease often triggered by AR. The condition is an acute or chronic inflammation of the nasal sinuses, the hollow cavities around the eyes and behind the nose. More than 15 percent of the U.S. population has this condition.
Eczema (atopic dermatitis). Eczema often happens when the allergen such as animal dander touches the skin. For 80 percent of people with the condition, symptoms started in childhood. More than 50 percent of sufferers develop asthma.
Hives (uritcaria) are caused by allergens such as foods, dust mites, pollens and animals. Medications include penicillin, sulfa, anti-seizure drugs, phenobarbital and aspirin.
Contact dermatitis. When skin contacts certain substances, usually a chemical, this condition may happen. Reaction may take hours or days to develop, as with poison ivy.
Allergic conjunctivitis is the most common eye allergy, occurring when airborne allergens get into the eye, irritating thin membranes covering the eyelids and the exposed surface of the eyes. Allergic conjunctivitis is in two forms: seasonal and perennial. The seasonal version is much more common, and is related to exposure to grass, tree and weed pollens and molds. The perennial form lasts throughout the year and can be triggered by dust mites, feathers or animal dander.
Food allergies may have severe and possibly life-threatening reactions, although this is not true for all people with food allergies. The most common triggers are cow's milk, eggs, peanuts, wheat, soy, fish, shellfish and tree nuts. Food allergies are more common in children and are increasing in prevalence. However, many children may outgrow certain allergies.
Anaphylaxis. Anaphylaxis is a rare, potentially fatal allergic reaction affecting many parts of the body at once. A trigger may be an insect sting, a food, such as nuts or shellfish, or medications. Usually the reaction is immediate and progresses within minutes. You must get to a hospital right away. About 150 to 200 people in the United States die each year from food-related anaphylaxis, the Food Allergy & Anaphylaxis Network reports.

Prognosis
Allergies do not have a cure, but they can be controlled effectively with good prevention, awareness of your triggers and knowing how to manage symptoms if they happen. If you cannot deal effectively with allergies on your own, a doctor can help.s

Do You Have Hay Fever?
If you have the following symptoms and they seem to come every spring or fall, you may have hay fever:
Congestion.
Sneezing.
Itching and dripping nose.
Itchy, watery eyes.