Atopic dermatitis is characterized by chronically itchy, superficially inflamed skin, often accompanied by allergic symp­toms such as hay fever and asthma. It also occurs in patients whose families have histories of allergies. The disease frequently occurs on the face and at or near the elbows and knees. Although atopic derma­titis is associated with allergies, the skin lesions do not have an allergic mechanism in most patients. Symptoms are commonly worse during the cold part of the year and are aggravated by contact irritants. In some people, skin symptoms appear to be aggravated in part by cer­tain foods or by exposure to inhaled allergens such as pollen.

В cells, or В lymphocytes, are lymphocytes derived from the bone marrow. They are involved in the production of antibodies.

Farmer’s lung is a form of allergic lung disorder caused by exposure to moldy hay. See also Hypersensitivity Pneu­monitis.

Grasses are widely distributed and include more than 4,000 species. Most of the wild native grasses produce very little pollen, leaving few introduced species as the major source of pollen. In the South, grass pollen may be encountered during most of the year. In colder regions, the season extends from mid-May to the end of July. In the Northeast, the bulk of the grass pollen is derived from blue-grass, timothy, orchard, and redtop. Velvet grass and rye grass are common on the Pacific coast. Bermuda is a dominant lawn and field grass in the southern states. Recently, Bahia grass has also become an important source of grass pollen in the South. Grasses are the major cause of hay fever in the United States during the summer.

The insects responsible for allergic reactions are almost exclusively in the class Hymenoptera. The most important are yellow jackets, hornets, wasps, honeybees, and fire ants. Allergy to insects may affect individuals who are either allergic (atopic) or non-allergic to common pollens and molds. Reactions may be local and are characterized by swelling, redness, and pain, or they may be systemic. Examples of systemic reactions are: generalized urticaria (hives); bronchospasm (asthma-like symptoms); throat swelling; and in some patients, low blood pressure. Severe systemic (anaphylactic) reactions may be fatal if not treated promptly. Anaphylactic reactions are mediated by allergic antibodies directed against components of the insect venom. Following the sting and penetration of the venom, the venom antigens react with the allergic antibodies present on the surface of mast cells and basophils. These antigen-antibody reactions initiate a series of intracellular events that culminate in the release of several active chemicals such as histamine. The action of these com­pounds on the various tissues is responsible for the symptoms of allergy in a person. The presence of significant amounts of allergic antibodies directed against insect venom can be demonstrated in an individual by skin tests or by a laboratory test, using the individual’s serum; the test is known as a radioallergosorbent (RAST) test. See RAST. In recent years the treatment of highly allergic patients has been improved significantly with the introduction of better allergenic materials for diagnosing and treating insect allergy. Today, it is recommended that patients with a clinical history of severe ana­phylactic reactions to insect venom, and high levels of allergic antibodies, be treated by immunotherapy, starting with small doses of venom extract and increasing the dosage. Following are the main characteristics of insects that cause allergic reactions: Yellow Jackets Bands of yellow and black around the thorax and abdomen. The head is yellow and black. Yellow jackets are second only to hornets in aggressiveness and are frequently seen near garbage and picnic areas. Their nests are large and are usually located in the ground, often in tall grass between the walls of buildings or under stones. Hornets Black body with white markings on the thorax and abdomen. The face, excluding the top of the head, is also white. Hornets may be difficult to distinguish from yellow jackets, since they often have yellow markings. Hornets’ nests are large, gray, or brown and are located in tree branches, under the eaves of houses, or against a wall, usually more than four feet above the ground. Hornets are considered the most aggressive of the major stinging insects, sometimes attacking without apparent provocation. Wasps Black or brown with yellow or white stripes and a fusiform (tapering toward each end) abdomen. The wasp can be distinguished from the yellow jacket and the hornet by its thin waist, which joins the abdomen and the thorax. Wasps are not as excitable as the other stinging insects; they usually sting only if touched or brushed while in flight around the nest. They build their nests in trees or around shelters such as the eaves of houses or porches. The nest is usually small and contains thirty to sixty insects. Honeybees Easily recognized by their small, stocky, brown, yel­low, or black bodies with a round abdomen. Probably the best-known insect, honeybees are found most frequently in artificial hives, but the colonies may be found in natural nests inside the trunks of trees, under floorboards, and in other enclosed areas. The stinger differs from that of the yellow jacket, wasp, or hornet, in that it cannot be used repeatedly (the stinging apparatus is severed from the body after the sting, the stinger is left in the skin, and the bee dies). Fire Ants Introduced from South America during the 1920s, fire ants are now well established in several Southern states. The venom of the fire ant differs from that of hymenoptera insects, in that it can produce severe local reactions or systemic anaphylactic reactions. The insect attacks by biting to secure itself, then inserts its stinging apparatus, which contains the venom. Highly sensitive patients with high levels of allergic antibodies against the venom of the fire ant are usually treated with immunotherapy. Someone who is sensitive to insect bites should avoid : 1. Standing in the direct path of a flight of insects 2. Contact with a nest 3. Throwing objects at a nest 4. Mowing the lawn 5. Planting flowers 6. Trimming hedges or shrubs 7. Painting houses 8. Handling trash or garbage cans 9. Walking barefoot outdoors 10. Cooking or eating outdoors 11. Using scented preparations such as perfume, hairsprays, colognes, or deodorants 12. Wearing brightly colored clothing with flower patterns

A mediator is a chemical substance that attracts or acti­vates other parts of the immune system—for example, histamine..

T cells are white blood cells produced by the bone marrow and processed by the thymus. Also called T lymphocytes, these cells are responsible for cellular immunity and delayed hypersensitivity. There are many known subtypes of T cells, with varying functions (for example, "killer," "helper," "suppressor") that either enhance or suppress immune responses.

Over the years, atopy (from the Greek atopia, for "uncom-monness") has been given various meanings. Broadly, it is a group of allergic diseases—mainly allergic rhinitis, allergic asthma, and some forms of eczema—that have some characteristics in common. Among these characteristics are a frequent, positive family history of similar allergic illnesses and the presence of increased serum levels of allergic antibodies to such common environmental allergens as pollen, molds, house dust, and animal danders. The most common feature of atopy in atopic patients is the tendency to develop allergic antibodies to common inhalant substances as a result of natural ex­posure. This tendency is now believed to be genetically determined.

The rebound phenomenon is an allergy con­dition in which prolonged use of a drug or drugs (such as nasal vaso­constrictor sprays for hay fever treatment) creates a need for more and more of the drug(s), while the desired effect steadily decreases.

The RAST (for radioallergosorbent test) is used to detect and measure allergic antibodies against an antigen in a person’s serum. It is more specific than skin tests but is less sensitive and more ex­pensive. Currently it is being used in certain patients for whom skin tests are technically difficult, such as those with severe skin diseases and young children or infants.