CD antigen

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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.
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 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.
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 B lymphocytes to produce antibodies without the participation of T lymphocytes. See also T-dependent antigen.
tumor antigen T antigen (def. 1).
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.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.


(ant'i-jen) [ anti- + -gen]
Any substance capable of eliciting an immune response or of binding with an antibody. Cellular antigens are proteins or oligosaccharides that mark and identify the cell surface as self or nonself. Cell surface antigens can stimulate the production of antibodies by B lymphocytes and cytotoxic responses by white blood cells, e.g., granulocytes, monocytes, and lymphocytes.

Antigens on the body's own cells are called autoantigens. Antigens on all other cells are called foreign antigens. Matching certain types of tissue antigens is important for the success of an organ transplant. Inflammation occurs when neutrophils, monocytes, and macrophages encounter an antigen from any source during bodily injury. The antigen may be foreign or an autoantigen that has been damaged and therefore appears to be foreign. Reactions to antigens by T and B cells are part of the specific immune response. See: autoantigen; cytokine; histocompatibility locus antigen.

allogeneic antigen

An antigen that occurs in some individuals of the same species. Examples are the human blood group antigens.

alpha-fetoprotein antigen

See: alpha-fetoprotein.

Australia antigen

, Australian antigen
A term formerly used for hepatitis B surface antigen.

bladder tumor antigen

Abbreviation: BTA
A protein released into the urine by malignant cells in the bladder, studied as a possible marker of cancer of the urinary bladder. Because of the low prevalence of bladder cancer in the population at large, and the low positive predictive value of the test, the U.S. Preventive Services Task Force (2006) discouraged health care professionals and patients from using this screening test.

cancer antigen

Abbreviation: CA
A protein or carbohydrate that is either expressed by cancerous cells but not by healthy cells or is expressed by cancerous cells in much greater concentrations than by healthy cells. Cancer antigens are used in clinical medicine to screen body fluids for tumors or to follow the response of tumors to treatment. Since they stimulate the immune response, they are also used in the manufacture of antitumor vaccines. See: table
Note: several antigens on this list also detect benign diseases and conditions.
Antigen name or designationAbbreviationThe tumor it detects
Alpha-fetoproteinAFPNonseminomatous germ cell tumor
CA 15–3Breast cancer
CA 19–9Pancreatic cancer
CA 50Gastrointestinal tract tumors
CA 125Ovarian/peritoneal cancer
Carcinoembryonic antigenCEAGastrointestinal tract tumors and tumors of solid internal organs
Human chorionic gonadotropinHCGNonseminomatous germ cell tumors; choriocarcinoma
Microglobulin-beta 2 subunitb2–MMultiple myeloma
Neuron-specific enolaseNSEBroad variety of cancers, including small cell carcinoma of lung
NY-BR-40 and othersBreast cancer
Prostate specific antigenPSAProstate cancer
Urinary tumor associated antigenUTAAMelanoma

carcinoembryonic antigen

Abbreviation: CEA
A molecular marker found on normal fetal cells and in the bloodstream of patients with cancers of the colon, breast, lung, and other organs. Assays for CEA are used both to monitor the effectiveness of treatments for cancer and to provide prognostic information to patients.

CD antigen

See: cluster of differentiation

class I antigen

Any of the major histocompatibility molecules present on almost all cells except human red blood cells. These antigens are important in the rejection of grafts and transplanted organs.

class II antigen

Any of the major histocompatibility molecules present on immunocompetent cells.

cross-reacting antigen

An antigen having the ability to react with more than one specific antibody.

D antigen

The protein marker in the Rh group of antigens that stimulates the greatest immune response. See: Rh blood group.

H antigen

A flagellar protein present on the surface of some enteric bacilli such as Escherichia coli. The antigen is important in classifying these bacilli.

hepatitis antigen

The original term for the Australian antigen, now called hepatitis B surface antigen (HBsAg). Its discovery made possible the differentiation of hepatitis B from other forms of viral hepatitis.

hepatitis B core antigen

Abbreviation: HBcAg
A protein marker found on the core of the hepatitis B virus (HBV). HBV antigen does not circulate in the blood but is found only in liver cells infected by HBV. HBcAg stimulates the production of a protective antibody, immunoglobulin M (IgM-anti-HBc), which appears in the blood shortly before the onset of symptoms. Tests for this antibody are used with other blood tests in the diagnosis of acute and chronic hepatitis B infection. During the convalescent stage of hepatitis B infection, IgM anti-HBc is replaced by another antibody, IgG anti-HBc, which remains in the blood for years. See: hepatitis B e antigen; hepatitis B surface antigen.

hepatitis B e antigen

Abbreviation: HBeAg
A polypeptide from the hepatitis B viral core that circulates in the blood of infected people and indicates that the patient is highly infectious. It is released when viral DNA is actively replicating.

hepatitis B surface antigen

Abbreviation: HBsAg
The glycoprotein found on the surface of the hepatitis B viral envelope. It is the first marker of infection with the hepatitis B virus. If HBsAg is still found in blood samples 6 months after infection with the virus, chronic and potentially contagious infection with hepatitis B is present. See: hepatitis B core antigen; hepatitis B e antigen.

hepatitis C core antigen

A protein released by the hepatitis C virus (HCV) into the bloodstream of infected patients. Because hepatitis C core antigen is detectable in the blood before HCV antibodies are produced, it can be used as a marker of early infection, e.g., in donated blood or plasma. It can also measure the response of HCV infection to treatment protocols; antigen levels drop with effective treatment.

histocompatibility locus antigen

Abbreviation: HLA
Any of the multiple antigens present on all nucleated cells in the body that identify the cells as self. Immune cells compare these antigens to foreign antigens, which do not match the self and therefore trigger an immune response. These markers determine the compatibility of tissue for transplantation.

They are derived from genes at seven sites (loci) on chromosome 6, in an area called the major histocompatibility complex (MHC); each histocompatibility antigen is divided into one of two MHC classes.

In humans, the proteins created in the MHC are called human leukocyte antigens (HLA) because these markers were originally found on lymphocytes. Each gene in the MHC has several forms or alleles. Therefore, the number of different histocompatibility antigens is very large, necessitating the identification and matching of HLAs in donors and recipients involved in tissue and organ transplantation. (The identification of HLAs is called tissue typing.)

The identification of HLA sites on chromosome 6 has enabled researchers to correlate the presence of specific histocompatibility and certain autoimmune diseases (e.g., insulin-dependent diabetes mellitus, multiple sclerosis, some forms of myasthenia gravis, rheumatoid arthritis, and ankylosing spondylitis).

Synonym: human leukocyte antigen See: major histocompatibility complex

human leukocyte antigen

Histocompatibility locus antigen.

H-Y antigen

A histocompatibility antigen located on the cell membrane. It has a primary role in determining the sexual differentiation of the male embryo.

K antigen

A capsular antigen present on the surface of some enteric bacilli. The antigen is important in classifying these bacilli.

lymphogranuloma venereum antigen

An antigen used in a skin test for lymphogranuloma venereum.

mumps skin test antigen

A standardized suspension of sterile formaldehyde-inactivated mumps virus. It is used in diagnosing mumps.

nuclear antigen

An antigen present in the cells of patients with certain types of connective tissue disorders. Corticosteroids can be very helpful in treating patients with high concentrations of nuclear antigen.

O antigen

A surface antigen of some enteric bacilli. The antigen is important in classifying these bacilli.

oncofetal antigen

An antigen that is normally expressed in the fetus and may reappear in the adult in association with certain tumors. Examples include alpha-fetoprotein and carcinoembryonic antigens.
Synonym: oncofetal protein

onconeural antigen

An antigen found on the surface of cancer cells that closely resembles antigens found on nerve cells. Antibodies formed by immune cells against onconeural antigens cause paraneoplastic syndromes.

p24 antigen

The core protein of the human immunodeficiency virus (HIV). The presence of p24 antigen in the blood is a marker of uncontrolled HIV replication. p24 antigenemia is encountered in the acute retroviral syndrome before host immune response and in advanced acquired immunodeficiency syndrome when the immune system has been destroyed. When p24 antigen is detected in the blood, the HIV viral load is high and the person is highly infectious.

proliferating cell nuclear antigen

Abbreviation: PCNA
A protein complex released by cells actively synthesizing DNA. In the blood, PCNAs can be used as markers of disease activity in autoimmune and inflammatory illnesses, malignancies, and other conditions marked by rapid cell replication.

prostate-specific antigen

Abbreviation: PSA
A nonspecific marker of abnormalities in the prostate gland, including prostatic infection, inflammation and prostate cancer. PSA circulates in the blood and can be detected by blood tests. PSA levels have been used to screen for prostate cancer, but are neither sensitive nor specific for detecting the disease.


Elevations in PSA levels may prompt invasive testing (such as with biopsies) that may detect indolent cancers as well as aggressive ones. Side effects of prostate biopsy include urinary incontinence and erectile dysfunction.
See: prostate cancer.

protective antigen

The protein made by Bacillus anthracis, which binds to cell membranes and allows the lethal components of anthrax toxin to enter and kill cells.

soluble antigen

An antigen dissolved in a liquid. A soluble antigen is recognized by B lymphocytes but cannot be detected by T lymphocytes until it has been processed by an antigen-presenting cell. See: T cell.

T-dependent antigen

An antigen that can stimulate an antibody response only in the presence of helper T cells.

thymus dependent antigen

Any of the foreign antigens that require B lymphocyte stimulation by T cells before production of antibodies and memory cells can occur.

thymus independent antigen

Any of the foreign antigens capable of stimulating B cell activation and the production of antibodies without T cell interaction. Most of these antibodies fall into the IgM class. A few memory cells are created.

T independent antigen

Either of two types of antigens that stimulate B cell production of antibodies without the presence of T cells. TI-1 antigens (e.g., lipopolysaccharides from gram-negative organisms) stimulate production of both specific (monoclonal) and nonspecific (polyclonal) antibodies and promote the release of cytokines from macrophages that enhance the immune response. TI-2 antigens, which result in monoclonal antibody production, may require the presence of cytokines. See: B cell; T cell

transplantation antigen

The commonly used term for any of the histocompatibility antigens that cause the immune system of one individual to reject transplanted tissue.

tumor-specific antigen

An antigen produced by certain tumors. It appears on the tumor cells but not on normal cells derived from the same tissue.
antigenic (-jen'ik) antigenically (-i-k(a-)le) antigenicity (-je-nis'i-te)
Medical Dictionary, © 2009 Farlex and Partners