antigen

antigen:

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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Antigen

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 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 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 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 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 Hypersensitivity, Immunology

antigen

[′an·tə·jən] (immunology) A substance which reacts with the products of specific humoral or cellular immunity, even those induced by related heterologous immunogens.

antigen

a substance that stimulates the production of antibodies

Antigen

antigen

[an´tĭ-jen] any substance capable, under appropriate conditions, of inducing a specific immune response" >immune response and reacting with the products of that response; that is, with specific antibody or specifically sensitized lymphocytes" >T lymphocytes, or both. Antigens may be soluble substances, such as toxins and foreign proteins, or particulates, such as bacteria and tissue cells; however, only the portion of the protein or polysaccharide molecule known as the determinant" >antigenic determinant combines with antibody or a specific receptor on a lymphocyte. Abbreviated Ag. See also immunity. adj., adj antigen´ic.allogeneic antigen one occurring in some but not all individuals of the same species, e.g., histocompatibility antigens and human blood group antigens; called also isoantigen.antigen-antibody reaction the reversible binding of antigen to homologous antibody by the formation of weak bonds between antigenic determinants on antigen molecules and antigen binding sites on immunoglobulin molecules.blood-group a's erythrocyte surface antigens whose antigenic differences determine blood groups.cancer antigen 125 (CA 125) a glycoprotein antigen found in normal adult tissues such as the epithelium of the fallopian tubes, the endometrium, the endocervix, the pleura, and the peritoneum. Elevated levels are seen in association with epithelial ovarian carcinomas, particularly nonmucinous tumors, as well as with some other malignancies, various benign pelvic disorders, tuberculosis, and cirrhosis.carcinoembryonic antigen (CEA) an oncofetal glycoprotein antigen originally thought to be specific for adenocarcinoma of the colon, but now known to be found in many other cancers and some nonmalignant conditions. Its primary use is in monitoring the response of patients to cancer treatment.CD antigen any of a number of markers" >cell-surface markers expressed by leukocytes and used to distinguish cell lineages, developmental stages, and functional subsets. Such markers can be identified by specific monoclonal antibodies and are numbered CD1, CD2, CD3, etc. (for cluster designation, according to how their specificity characteristics group together when analyzed by computer).CD4 antigen an antigen on the surface of helper T cells; the normal range of helper cells is 800 to 1200 per cubic mm of blood. The virus" >human immunodeficiency virus binds to this antigen and infects and kills T cells bearing this antigen, thus gradually destroying the body's ability to resist infection. CD4 can be administered in a soluble form to increase the amount of it in the circulation and interfere with the ability of HIV to affect CD4 antigens on the cell.class I a's major antigens" >histocompatibility antigens found on virtually every cell, human erythrocytes being the only notable exception; they are the classic histocompatibility antigens recognized during graft rejection.class II a's major antigens" >histocompatibility antigens found only on immunocompetent cells, primarily B lymphocytes and macrophages.conjugated antigen antigen produced by coupling a hapten to a protein carrier molecule through covalent bonds; when it induces immunization, the resultant immune response is directed against both the hapten and the carrier.cross-reacting antigen 1. one that combines with antibody produced in response to a different but related antigen, owing to similarity of antigenic determinants.2. identical antigens in two bacterial strains, so that antibody produced against one strain will react with the other.extractable nuclear a's ENA; protein antigens, not containing DNA, that are extractable from cell nuclei in phosphate-buffered saline; anti-ENA antibodies are a component of the antinuclear antibodies occurring in systemic lupus erythematosus and other connective tissue diseases.flagellar antigen H antigen.Forssman antigen a antigen" >heterogenetic antigen discovered in guinea pig tissues, capable of lysing sheep erythrocytes in the presence of complement. It is found usually in animal organs but occasionally in blood, and induces formation of an antibody (Forssman antibody, a type of heterophile antibody) only when combined with protein or hog serum. Davidsohn's Differential Test was historically used to differentiate between the heterophile sheep agglutinins in human serum that were due to Forssman antigen and those due to infectious mononucleosis; this is based upon the fact that boiled guinea pig kidney will absorb heterophile sheep cell agglutinins produced by Forssman antigen, but not those produced by infectious mononucleosis.H antigen (Ger. Hauch, film), the antigen that occurs in the flagella of motile bacteria.hepatitis B core antigen (HBcAg) a core protein antigen of the hepatitis B virus present inside complete virions (Dane particles) and in the nuclei of infected hepatic cells, indicating the presence of reproducing hepatitis B virus. The antigen is not present in the blood of infected individuals, but antibodies against it appear during the acute infection; they do not protect against reinfection.hepatitis B e antigen (HBeAg) an antigen of hepatitis B virus sometimes present in the blood during acute infection, usually disappearing afterward but sometimes persisting in chronic disease. Anti-HBe antibodies appear transiently during convalescence; they do not protect against reinfection.hepatitis B surface antigen (HBsAg) one present in the serum of those infected with hepatitis B, consisting of the surface coat lipoprotein of the hepatitis B virus. Tests for serum HbsAg are used in the diagnosis of hepatitis B and in testing blood products for infectivity.heterogeneic antigen xenogeneic antigen.heterogenetic antigen (heterophil antigen) (heterophile antigen) one capable of stimulating the production of antibodies that react with tissues from other animals or even plants.histocompatibility a's genetically determined isoantigens present on the cell membranes of nucleated cells of most tissues, which incite an immune response when grafted onto a genetically disparate individual and thus determine the compatibility of tissues in transplantation. Major histocompatibility antigens are those that belong to the complex" >major histocompatibility complex, which in humans contains the hla antigens. Minor histocompatibility antigens are those that can cause delayed tissue rejection.HLA a's (human leukocyte a's) see hla antigens.H-Y antigen a minor antigen" >histocompatibility antigen present in all tissues of normal males and coded for by a structural gene on the short arm of the Y chromosome; it is thought to promote the differentiation of indifferent gonads into testes, thus determining male sex.isogeneic antigen an antigen carried by an individual which is capable of eliciting an immune response in genetically different individuals of the same species, but not in an individual bearing it.K antigen a bacterial capsular antigen, a surface antigen external to the cell wall.lymphogranuloma venereum antigen a sterile suspension of Chlamydia lymphogranulomatis; used as a dermal reactivity indicator.M antigen a type-specific antigen that appears to be located primarily in the cell wall and is associated with virulence of Streptococcus pyogenes.mumps skin test antigen a sterile suspension of mumps virus; used as a dermal reactivity indicator.nuclear a's the components of cell nuclei with which antinuclear antibodies react.O antigen (Ger. ohne Hauch, without film), the antigen that occurs in the bodies of bacteria.oncofetal antigen a gene product that is expressed during fetal development, but repressed in specialized tissues of the adult and that is also produced by certain cancers. In the neoplastic transformation, the cells dedifferentiate and these genes can be derepressed so that the embryonic antigens reappear. Examples are alpha-fetoprotein and carcinoembryonic antigen.organ-specific antigen any antigen that occurs exclusively in a particular organ and serves to distinguish it from other organs. Two types of organ specificity have been proposed: (1) first-order or tissue specificity is attributed to the presence of an antigen characteristic of a particular organ in a single species; (2) second-order organ specificity is attributed to an antigen characteristic of the same organ in many, even unrelated, species.partial antigen an antigen that does not produce antibody formation, but gives specific precipitation when mixed with the antibacterial immune serum.pollen antigen the essential polypeptides of the pollen of plants extracted with a suitable menstruum, used in diagnosis, prophylaxis, and desensitization in hay fever.antigen presentation the presentation of ingested antigens on the surface of macrophages in close proximity to antigens" >histocompatibility antigens. Some populations of lymphocytes" >T lymphocytes can only be triggered by antigens that are presented in this way. Thus macrophages play a role in inducing cell-mediated immunity.private a's antigens of the low-frequency blood groups, so called because they are found only in members of a single kindred.prostate-specific antigen (prostatic specific antigen) an antigen that is elevated in all patients with prostatic cancer and in some with an inflamed prostate gland.public a's antigens of the high-frequency blood groups, so called because they are found in many persons.self antigen an autoantigen, a normal constituent of the body against which antibodies are formed in autoimmune disease.sequestered a's the cellular constituents of tissue (e.g., the lens of the eye and the thyroid) sequestered anatomically from the lymphoreticular system during embryonic development and thus thought not to be recognized as “self.” Should such tissue be exposed to the lymphoreticular system during adult life, an autoimmune response would be elicited.somatic a's antigens, usually cell surface antigens, of the body of a bacterial cell, in contrast to flagellar or capsular antigens.T antigen 1. any of several antigens, coded for by the viral genome, associated with transformation of infected cells by certain DNA tumor viruses. Called also tumor antigen.2. an antigen present on human erythrocytes that is exposed by treatment with neuraminidase or contact with certain bacteria.see CD a.T-dependent antigen one that requires the presence of helper T cells to stimulate antibody production by B cells; most antigens are T-dependent.T-independent antigen an antigen that can trigger lymphocytes" >B lymphocytes to produce antibodies without the participation of lymphocytes" >T lymphocytes. See also T-dependent antigen.tumor antigen T antigen (def. 1).tumor-specific antigen.tumor-specific antigen (TSA) any cell-surface antigen of a tumor that does not occur on normal cells of the same origin.V antigen (Vi antigen) an antigen contained in the sheath of a bacterium, as Salmonella typhosa (the typhoid bacillus), and thought to contribute to its virulence.xenogeneic antigen an antigen common to members of one species but not to members of other species; called also heterogeneic antigen.

an·ti·gen (Ag),

(an'ti-jen), Any substance that, as a result of coming in contact with appropriate cells, induces a state of sensitivity or immune responsiveness and that reacts in a demonstrable way with antibodies or immune cells of the sensitized subject in vivo or in vitro. Modern usage tends to retain the broad meaning of antigen, employing the terms "antigenic determinant" or "determinant group" for the particular chemical group of a molecule that confers antigenic specificity.
See also: hapten. Synonym(s): immunogen [anti(body) + G. -gen, producing]

antigen

(ăn′tĭ-jən) also

antigene

(-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.

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

antigen

A molecule that usually has a molecular weight of > 1 kD, is a protein, often foreign (i.e., non-self), and which is capable of evoking a specific immune response (antibody production).

antigen

A molecule that usually has a molecular weight of > 1 kD, is a protein, often foreign–ie, non-self, which is capable of evoking a specific immune response, antibody production. See A antigen, Acquired B antigen, Alloantigen antigen, Antigenicity, Australia antigen, Autoantigen, B antigen, Bladder tumor antigen, Cancer-associated antigen, CENP antigen, Cromer-related antigen, Early antigen, Eclipsed antigen, Epithelial membrane antigen, Epstein-Barr nuclear antigen, Epstein-Barr viral capsid antigen, Extractable nuclear antigen, GAS, H antigen, Hepatitis surface antigen, High frequency antigen, H-Y antigen, Leukocyte common antigen, Lewis Y antigen, Low-frequency antigen, Mls antigen, Myeloid antigen, O antigen, Oncofetal antigen, P antigen, p24 antigen, Platelet antigen, Private antigen, PSA-prostate-specific antigen, Proliferating cell nuclear antigen, PRP antigen, Public antigen, Rh antigen, S antigen, Self antigen, Sm antigen, Soluble antigen, Squamous cell carcinoma antigen, SS-A antigen, SS-B antigen, Superantigen, TA-90, TAG-72, Targett antigen, Tumor-associated antigen, U antigen, Vi antigen, Xga antigen. Cf Antibody, Hapten.

an·ti·gen

(Ag) (an'ti-jen) Any substance that, as a result of coming in contact with target cells, induces a state of sensitivity or immune responsiveness after a latent period (days to weeks) and that reacts in a demonstrable way with antibodies or immune cells of the sensitized subject in vivo or in vitro. Modern usage tends to retain the broad meaning of antigen, employing the terms "antigenic determinant" or "determinant group" for the particular chemical group of a molecule that confers antigenic specificity.
See also: hapten
Synonym(s): immunogen. [anti(body) + G. -gen, producing]

antigen

Any molecule recognized by the immune system of the body as signalling ‘foreign’, and which will provoke the production of a specific ANTIBODY. Antigens include molecules on the surfaces of infective viruses, bacteria and fungi, pollen grains and donor body tissue cells.

antigen (Ag)

a complex molecule (usually a protein or carbohydrate) that, when introduced into the body, induces an IMMUNE RESPONSE which includes the production of specific ANTIBODIES. Antigens can be toxins (as in snake venom) or molecules on cell surfaces (e.g. A/B antigens on red blood cells).

Antigen

A substance that stimulates the immune system to manufacture antibodies (immunoglobulins). The function of antibodies is to fight off intruder cells, such as bacteria or viruses, in the body. Antigens stimulate the blood to fight other blood cells that have the wrong antigens. If a person with blood type A is given a transfusion with blood type B, the A antigens will fight the foreign blood cells as though they were an infection.Mentioned in: AIDS, AIDS Tests, Allergic Bronchopulmonary Aspergillosis, Allergic Rhinitis, Allergies, Allergy Tests, Ataxia-Telangiectasia, Blood Typing and Crossmatching, Bone Marrow Transplantation, Botulinum Toxin Injections, Coccidioidomycosis, Coombs' Tests, Cryptococcosis, Delayed Hypersensitivity Skin Test, Elephantiasis, Erythroblastosis Fetalis, Escherichia Coli, Giardiasis, Goodpasture's Syndrome, Hemorrhagic Fevers, Hepatitis, Autoimmune, Hepatitis B, Histoplasmosis, Immune Complex Test, Immunoelectrophoresis, Immunosuppressant Drugs, Legionnaires' Disease, Liver Transplantation, MALT Lymphoma, Multiple Myeloma, Prostate-Specific Antigen Test, Rheumatic Fever, Sore Throat, Streptococcal Antibody Tests, Throat Culture, Tissue Typing, Transfusion, Tuberculin Skin Test, Vasculitis, Waldenström's Macroglobulinemia

antigen

Any substance that can stimulate an immune response in the body and can react with the products of that response, that is, with specific antibodies or specifically sensitized T lymphocytes, or both. Antigens include bacteria, foreign substance (e.g. dust mite, grass, pollen of trees), toxins and viruses. See allergic reactions; sensitization.

an·ti·gen

(an'ti-jen) Any substance that, as a result of coming in contact with appropriate cells, induces a state of sensitivity or immune responsiveness and reacts in a demonstrable way with antibodies or immune cells of the sensitized subject in vivo or in vitro. [anti(body) + G. -gen, producing]

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antigen (Ag)

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Words related to antigen

noun any substance (as a toxin or enzyme) that stimulates an immune response in the body (especially the production of antibodies)

Related Words