An allergy is an abnormal reaction to an ordinarily harmless sub­stance or substances. These substances, called allergens, may be in­haled, swallowed, injected, or contacted by the skin.

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There are many possible allergens. For example: pollen, mold, and house dust; animal dander (skin shed by dogs, cats, horses, rabbits, and so on) ; feathers (as in old-fashioned pillows); kapok, wool, dyes, and industrial chemicals; foods and medicines; and insect stings.

No. Allergy may develop in people of any age. Even infants some- times exhibit symptoms of allergy, and some people experience their first attacks of allergy in middle age or later. Nevertheless, children are more likely to develop allergies than are people later in life.

Allergic diseases or reactions can involve any part of the body. The parts, or systems, most frequently involved are the respiratory sys­tem, where an allergic reaction may take the form of hay fever (aller­gic rhinitis) and asthma, and the skin, where a reaction can take the form of allergic dermatitis or atopic eczema, contact dermatitis (for example, poison ivy), or hives (urticaria). An allergic disease may also be a factor in numerous other diseases. Hay fever is caused by allergy to the pollen of trees, grasses, weeds, or molds, or any combination of these. Depending on the section of the United States (unless otherwise indicated, this means the conti­nental United States) and the pollinating periods, it may occur in the spring, summer, or fall, and it may last until the first frost. The hay fever sufferer has spells of sneezing, itching, weeping eyes, running nose, and a burning sensation in the palate and throat. Allergic rhinitis is a general term that applies to anyone with nasal congestion, sneezing, and a running nose—all caused by allergies. This may be a seasonal problem, as with hay fever, or it may be a year-round problem caused by other allergens such as house dust, animal dander, and perhaps some foods. Because allergic rhinitis is frequently confused with sinusitis, patients with constant nasal symp­toms should be evaluated for allergies. Asthma is a condition characterized by coughing, wheezing, and difficulty breathing. It is frequently, but not invariably, associated with a family history of allergy. Any of the previously mentioned allergens can precipitate asthma attacks. Infections of the sinuses or the bron­chial tubes may also be an important factor. Asthma patients are adversely affected by such "nonspecific" factors as air pollutants, cigarette smoke, and fumes. Their own emotions sometimes come into play. Asthma may begin at any age. If neglected, it tends to recur and become chronic. Allergic dermatitis, or eczema, is a noncontagious, itchy rash that often occurs in the creases of the arms, legs, and neck, although it sometimes covers the entire body. The condition is frequently asso­ciated with allergies, and substances to which a person is sensitive can aggravate it. Foods are known to cause allergic dermatitis. A family history of allergy is thought to be a factor in its occurrence. Contact dermatitis is a rash caused by direct skin contact with any of a variety of substances—animal, plant, chemical, or mineral. The most common cause is poison ivy. Urticaria, or hives, is an outbreak on the skin of itchy welts of vary­ing size. When the welts are large and invade deeper tissues, they are called angioedema. They may develop on the face, lips, tongue, throat, eyes, ears, or even internally. Allergies to food or drugs (espe­cially penicillin) are well-known causes, but hives may also result from an underlying disease, or the disease may occur after emotional stress. Sometimes the exact cause cannot be determined.

When an allergen is absorbed into the bloodstream, it stimulates certain small white blood cells called lymphocytes to produce special substances known as allergic antibodies. These antibodies react with the allergen and produce allergic inflammation and irritation in par­ticularly sensitive areas of the body, such as the nose, eyes, lungs, or digestive system. This sensitivity is not present at first contact with the allergen; instead, it may develop after repeated exposure. For exam­ple, a new cat may not cause allergy until it has been living in a house for many months. Eventually, a susceptible person becomes sensitized and develops a stuffy nose and sneezing or wheezing on further con­tact with the cat.

Statistics compiled by the National Institute of Allergy and Infec­tious Diseases show that thirty-five million people suffer from allergy and that nine million of this group have asthma.