antigen

(ăn'tĭ-jən) also an·ti·gene (-jēn')
n.
A substance that when introduced into the body stimulates the production of an antibody. Antigens include toxins, bacteria, foreign blood cells, and the cells of transplanted organs.

antigenic an'ti·gen'ic (-jĕn'ĭk) adj.
antigenically an'ti·gen'i·cal·ly adv.
antigenicity an'ti·ge·nic'i·ty (-jə-nĭs'ĭ-tē) n.

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For more information on antigen, visit Britannica.com.

A substance that initiates and mediates the formation of the corresponding immune body, termed antibody. Antigens can also react with formed antibodies. Antigen-antibody reactions serve as host defenses against microorganisms and other foreign bodies, or are used in laboratory tests for detecting the presence of either antigen or antibody. See also Antibody; Antigen-antibody reaction.

A protein immunogen (any substance capable of inducing an immune response) is usually composed of a large number of antigenic determinants. Thus, immunizing an animal with a protein results in the formation of a number of antibody molecules with different specificities. The antigenicity of a protein is determined by its sequence of amino acids as well as by its conformation. Antigens may be introduced into an animal by ingestion, inhalation, sometimes by contact with skin, or more regularly by injection into the bloodstream, skin, peritoneum, or other body part.

With a few exceptions, such as the autoantigens and the isoantigens of the blood groups, antigens produce antibody only in species other than the ones from which they are derived. All complete proteins are antigenic, as are many bacterial and other polysaccharides, some nucleic acids, and some lipids. Antigenicity may be modified or abolished by chemical treatments, including degradation or enzymatic digestion; it may be notably increased by the incorporation of antigen into oils or other adjuvants. See also Isoantigen.

Bacteria, viruses, protozoans, and other microorganisms are important sources of antigens. These may be proteins or polysaccharides derived from the outer surfaces of the cell (capsular antigens), from the cell interior (the somatic or O antigens), or from the flagella (the flagellar or H antigens). Other antigens either are excreted by the cell or are released into the medium during cell death and disruption; these include many enzymes and toxins, of which diphtheria, tetanus, and botulinus toxins are important examples. The presence of antibody to one of these constituent antigens in human or animal sera is presumptive evidence of past or present contact with specific microorganisms, and this finds application in clinical diagnosis and epidemiological surveys. See also Botulism; Diphtheria; Toxin.

Microbial antigens prepared to induce protective antibodies are termed vaccines. They may consist of either attenuated living or killed whole cells, or extracts of these. Since whole microorganisms are complex structures, vaccines may contain 10 or more distinct antigens, of which generally not more than one or two engender a protective antibody. Examples of these are smallpox vaccine, a living attenuated virus; typhoid vaccine, killed bacterial cells; and diphtheria toxoid, detoxified culture fluid. Several independent vaccines may be mixed to give a combined vaccine, and thus reduce the number of injections necessary for immunization, but such mixing can result in a lesser response to each component of the mixture. See also Vaccination.

Allergens are antigens that induce allergic states in humans or animals. Examples are preparations from poison ivy, cottonseed, or horse dander, or simple chemicals such as formaldehyde or picryl chloride. See also Heterophile antigen; Hypersensitivity; Immunology.

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Any compound that is foreign to the body (e.g. bacterial, food, or pollen protein, or some complex carbohydrates) which, when introduced into the circulation, stimulates the formation of an antibody. See also adverse reactions to foods.

An antigen is a substance that is capable of inducing a specific immune response in the host into which it is introduced. The immune response is mediated via an immunoglobulin (protein) molecule, called an antibody, which is formed by B-lymphocytes and T-helper cells that are the basic ingredients of the host's immune system. "Antibody" is the generic name for any immunoglobulin thus produced, no matter how this occurs. Humans can produce many specific antibodies. This may be an active process by a healthy host in response to the challenge of exposure to a foreign antigen transmitted via the placenta or in maternal milk from mother to offspring, or it may be artificially induced by immunization with live attenuated organisms, killed organisms, or a protein derivative.

An antigen is an organic compound—a protein, polysaccharide or glycolipid. Sometimes it is an entire organ or tissue that has been transplanted into the host, which rejects it and attempts to destroy it. An antibody has the capacity to bind specifically to the (foreign) antigen and thereby neutralize it so it can be destroyed by the host's phagocytes.

The antibody is the basic ingredient of the host's defenses against infection. By measuring the concentration of specific antibodies in individuals and populations it is possible to determine levels of susceptibility and resistance to infection by specific pathogens. At the population level, this is called "sero-epidemiology."

An antigen is produced by living organisms, which evolve over generations and can therefore undergo subtle changes in protein composition, a process known as "antigen drift." A more sudden evolutionary change can lead to an abrupt change in protein structure, known as "antigen shift." Antigen drift renders an antibody less effective, and antigen shift makes an antibody ineffective in combating an antigen to which the host was immunized by exposure to the previous form of the antigen. Antigen drift and antigen shift account for recurrences of infection with viruses, such as those of the common cold and influenza.

(SEE ALSO: Epidemiology; Immunizations)

— JOHN M. LAST

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A substance (usually a protein or carbohydrate) that is identified by cells in the immune system as being foreign to the body, and which induces an immune response.

(an-ti-juhnz)

Substances that are foreign to the body and cause the production of antibodies. Toxins, invading bacteria and viruses, and the cells of transplanted organs can all function as antigens.

the capacity of an agent to stimulate the formation of specific antibodies to itself.

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The capacity to stimulate the production of antibodies or cell-mediated immune responses.

(an′tijen)
n

A substance, usually a protein, that elicits the formation of antibodies that react with it when introduced parenterally into an individual or species to which it is foreign.

For a list of words related to antigen, see:

• Diagnostic Terminology - antigen: foreign substance, usu. protein, against which body produces an antibody

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See words rhyming with "antigen."

  See crossword solutions for the clue Antigen.

Dansk (Danish)
n. - antigen

Nederlands (Dutch)
antigen (stof die afweersysteem activeert)

Français (French)
n. - antigène

Deutsch (German)
n. - (med.) Antigen

Ελληνική (Greek)
n. - (χημ.) αντιγόνο

Italiano (Italian)
antigene

Português (Portuguese)
n. - antígeno (m) (Med.)

Русский (Russian)
антиген

Español (Spanish)
n. - antígeno

Svenska (Swedish)
n. - antigen

中文（简体）(Chinese (Simplified))
抗原

中文（繁體）(Chinese (Traditional))
n. - 抗原

한국어 (Korean)
n. - 항원

日本語 (Japanese)
n. - 抗原

العربيه (Arabic)
‏(الاسم) مولد مضاد : ماده ينتج عن حقنهافي الجسم اجسام مضاده‏

עברית (Hebrew)
n. - ‮חומר זר (למשל רעל) המעורר תגובה חיסונית בגוף, מייצר נוגדנים, אנטיגן‬

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