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any substance capable, under appropriate conditions, of inducing a specific immune response and reacting with the products of that response; that is, with specific antibody or specifically sensitized 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 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 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 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 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 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.
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 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 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 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 histocompatibility antigens. Some populations of 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.
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 B lymphocytes to produce antibodies without the participation of T lymphocytes. See also T-dependent 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.
"virulence antigen," an external capsular antigen of enterobacteria formerly thought to be related to increased virulence.