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hypersensitive reaction of the body tissues of certain individuals to certain substances that, in similar amounts and circumstances, are innocuous to other persons. Allergens, or allergy-causing substances, can be airborne substances (e.g., pollens, dust, smoke), infectious agents (bacteria, fungi, parasites), foods (strawberries, chocolate, eggs, nuts and peanuts), contactants (poison ivy, chemicals, dyes), or physical agents (light, heat, cold). It is believed that a person who is hereditarily predisposed toward allergy produces, when sensitized, special weak types of antibodies, called reagins, that give little immune protection but cause local tissue damage during the antibody-antigen reaction (see immunityimmunity,
ability of an organism to resist disease by identifying and destroying foreign substances or organisms. Although all animals have some immune capabilities, little is known about nonmammalian immunity.
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). A sensitivity toward an allergen need not, however, lead to an allergy. A study reported in 2015 that young children with a peanut sensitivity were more likely to develop a peanut allergy if they avoided, instead of being regularly exposed to, peanuts, and in 2017 the U.S. National Institute of Allergy and Infectious Diseases recommended that children be exposed to foods with peanuts by around six months of age to reduce the likelihood of peanut allergy.

Allergens can affect the respiratory system, the reaction manifesting itself as asthma or hay fever, or they can affect the skin, causing wheals and rashes. Allergens may also act on the gastrointestinal tract, causing nausea and vomiting. Allergic reactions to substances injected into the bloodstream can cause violent and sometimes fatal reactions (see anaphylaxisanaphylaxis
, hypersensitive state that may develop after introduction of a foreign protein or other antigen into the body tissues. When an anaphylactic state exists, a second dose of the same protein (commonly an antibiotic such as penicillin, or certain insect venoms) will
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; serum sicknessserum sickness,
hypersensitive response that occurs after injection of a large amount of foreign protein. The condition is named for the serum taken from horses or other animals immunized against a particular disease, e.g., tetanus or diphtheria.
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). The best treatment of allergic reactions is prevention, i.e., elimination of the offending substances from the sensitive person's environment. If this is not possible, desensitization (i.e., deliberate production of the allergic reaction by injecting the allergen, after which the sufferer is no longer susceptible) is sometimes helpful. Antihistamine drugs may give temporary relief. See histaminehistamine
, organic compound derived in the body from the amino acid histidine by the removal of a carboxyl group (COOH). Although found in many plant and animal tissues, histamine is specifically important in human physiology because it is one of the chemicals released from
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Altered reactivity in humans and animals to allergens (substances foreign to the body that cause allergy) induced by exposure through injection, inhalation, ingestion, or skin contact. The most common clinical manifestations of allergy are hay fever, asthma, hives, atopic (endogenous) eczema, and eczematous skin lesions caused by direct contact with allergens such as poison ivy or certain chemicals.

A large variety of substances may cause allergies: pollens, animal proteins, molds, foods, insect venoms, foreign serum proteins, industrial chemicals, and drugs. Most natural allergens are proteins or polysaccharides of moderate molecular size (molecular weights of 10,000 to 200,000). Chemicals or drugs of lower molecular weight (haptens) have first to bind to the body's own proteins (carriers) in order to become fully effective allergens.

For the development of the hypersensitivity state underlying clinical allergies, repeated contact with the allergen is required. Duration of the sensitization period is usually dependent upon the sensitizing strength of the allergen and the intensity of exposure. Some allergens (for example, saliva, urine, and hair proteins of domestic animals) are more sensitizing than others. In most instances, repeated contact with minute amounts of allergen is required; several annual seasonal exposures to grass pollens or ragweed pollen usually occur before an overt manifestation of hay fever. On the other hand, allergy to cow milk proteins in infants can develop within a few weeks. When previous contacts with allergens have not been apparent (for example, antibiotics in food), an allergy may become clinically manifest even upon the first conscious encounter with the offending substance.

Besides the intrinsic sensitizing properties of allergens, individual predisposition of the allergic person to become sensitized also plays an important role. Clinical manifestations, such as hay fever, allergic asthma, and atopic (endogenous) dermatitis, occur more frequently in some families. In other clinical forms of allergy, genetic predisposition, though possibly present as well, is not as evident.

Exposure to sensitizing allergens may induce several types of immune response, and the diversity of immunological mechanisms involved is responsible for the various clinical forms of allergic reactions which are encountered in practice. Three principal types of immune responses are encountered: the production of IgE antibodies, IgG or IgM antibodies, and sensitized lymphocytes. See Antibody, Immunoglobulin

Diagnosis of allergic diseases encompasses several facets. Since many clinical manifestations of allergy are mimicked by nonallergic mechanisms, it is usually necessary to use additional diagnostic procedures to ascertain whether the person has developed an immune response toward the incriminated allergen. Such procedures primarily consist of skin tests, in which a small amount of allergen is applied on or injected into the skin. If the individual is sensitized, a local immediate reaction ensues, taking the form of a wheal (for IgE-mediated reactions), or swelling and redness occurs after several hours (for delayed hypersensitivity reactions). The blood may also be analyzed for IgE and IgG antibodies by serological assays, and sensitized lymphocytes are investigated by culturing them with the allergen.

Since the discovery of the responsible allergens markedly influences therapy and facilitates prediction of the allergy's outcome, it is important to achieve as precise a diagnosis as possible. Most tests indicate whether the individual is sensitized to a given allergen, but not whether the allergen is in fact still causing the disease. Since in most cases the hypersensitive state persists for many years, it may well happen that sensitization is detected for an allergen to which the individual is no longer exposed and which therefore no longer causes symptoms. In such cases, exposition tests, consisting of close observation of the individual after deliberate exposure to the putative allergen, may yield useful information.

The most efficient treatment, following identification of the offending allergen, remains elimination of allergen from the person's environment and avoidance of further exposure. This form of treatment is essential for allergies caused by most household and workplace allergens. See Antigen, Hypersensitivity

McGraw-Hill Concise Encyclopedia of Bioscience. © 2002 by The McGraw-Hill Companies, Inc.
The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



a distorted sensitivity or reaction of the organism to an allergen.

Allergens can be a variety of substances, from the very simplest (iodine and bromine) to complex proteins and nonproteins (polysaccharides, lipid-polysaccharide compounds, etc.); certain dyes; and certain medicines, including amidopyrine (pyramidone), butadione, barbiturates, chloral hydrate, digitalis preparations, morphine, quinine, sulfanilamides, and antibiotics.

Allergens which enter the body from the external environment are called exoallergens and may be infectious—for example, disease-bearing and nondisease-bearing microorganisms, their vital products, viruses, and products of their interaction with tissues—or noninfectious—for instance, house dust, animal hair, plant pollen, medicines, chemicals such as benzene, chloramine, and p-phenylendiamine, meat, fish, fruits, vegetables, berries, and milk.

Allergens can be tissues of the organism itself—that is, autoallergens, including the crystalline lens of the eye, the myelin of neural tissue, and thyroid and ovarian tissue. In some cases the allergens can be other body tissues—for example, cardiac tissue (in rheumatism and myocardial infarct), kidney (in nephritis), liver (in hepatitis), tissues damaged by the effects of burns, radiation or cold, pathogenic microbes, viruses, and also products of the interaction of microbes and viruses with human tissues. In response to the entry of an allergen into the body, the lymphatic tissues—the spleen and lymph nodes—produce antibodies which “neutralize” the allergen. Upon repeated entry of an allergen, antibody production increases sharply, as a result of which the interaction of allergen and antibody generates an active substance such as a histamine or bradykinin, which destroys the permeability of blood-bearing capillaries, produces tissue damage and inflammation, and affects neural receptors.

The symptoms of allergy—for instance, the formation of cutaneous blisters, bronchiospasms, and the development of inflammation—can arise within a few minutes or hours after contact with the allergen or after a longer period of up to several days in the case of tuberculin reaction, some types of eczema, and nettle rash. Allergies develop in different ways. The allergic reaction may develop extremely rapidly and be distinguished by its great severity—for example, anaphylactic shock—and is called hyperergy; a diminution in allergic activity is hypergy, while a complete absence of a response by the organism is called anergy. Anergy is considered positive when the allergic response to a disease is reduced through congenital or acquired immunity to the allergen, and negative when there is heavy intoxication and exhaustion of the organism from the infection—this can occur in tuberculosis and pneumonia. Sometimes the allergic state is caused by one allergen in relation to another—for example, a positive tuberculin reaction in children after a smallpox inoculation. The phenomenon is called para-allergy. The restoration of a specific allergic reaction after administration of a nonspecific irritant—for example, the restoration of the tuberculin reaction in tubercular patients after injection with scrub typhus vaccine—is called metallergy.

The common term allergy is used to denote increased sensitivity to an allergen. Allergens exist which almost always produce an allergy when administered by certain methods—for example, repeated injections of foreign proteins. In other cases allergy develops only in individual persons predisposed to it—for instance, allergic diasthesis, which is often hereditary. Allergic diasthesis is characterized by increased blood vascular permeability, which allows allergens to easily enter the blood and tissues of the patient via the respiratory organs, the digestive tract, and the like, resulting in an allergic response. It is known that many diseases have an allergy as their basis—including bronchial asthma, nettle rash, medicinal allergy, rheumatism, contact dermatitis, and transplant rejection reactions. In persons with allergic diasthesis various allergic diseases are often involved. The most severe allergic diseases—systemic lupus erythematosis, other collagen diseases, hemolytic anemia, certain forms of hemophilia, and irritations of the eye and thyroid gland—are caused by the action of autoallergens. In some diseases, especially infectious ones, allergic reactions accumulate during the course of the basic process. There is a resemblance to an allergic reaction in persons suffering from idiosyncrasies. This gives many authors the basis for linking idiosyncrasy with the allergic diseases.

Patients are treated with hormone preparations, antihistamines, salicylates, and so on. Prophylaxis calls for limiting or eliminating contact with allergens and treating diseases—especially chronic infections—which might lead to allergy.


Sovremennaia prakticheskaia allergologiia. Edited by A. D. Ado and A. A. Pol’ner. Moscow, 1963.
Zheltakov, M. M., and B. A. Somov. Allergiia k lekarstvennym veshchestvam. Moscow, 1968. (Bibliography, pages 344–88.)
Immunopathology. Edited by P. Grabar and P. Miescher. Basel-Stuttgart, 1963.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A type of antigen-antibody reaction marked by an exaggerated physiologic response to a substance that causes no symptoms in nonsensitive individuals. Also known as allergic reaction.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


a hypersensitivity to a substance that causes the body to react to any contact with that substance. Hay fever is an allergic reaction to pollen
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005