antigen

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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
References in periodicals archive ?
This study showed that CA 125 had 73% sensitivity and 78% specificity in predicting optimal cytoreduction, and other studies have yielded similar results.
It was believed that CA 125 is more elevated in females than males and ovaries are a natural source of it but studies have not supported this fact and gender usually has no direct association with raised levels of this marker.
On the other hand; the serum CEA and CA 125 levels were found to be significantly elevated in gastric cancer patients than in controls (pless than 0.001 for both) (Table-I).The serum tumor marker levels of patients didn't show any significant difference according to either T stage or N stage of the disease (pgreater than 0.05 for all markers) (Table-II).
Preoperatively, 35.2% (n = 117), 9.6% (n = 32), and 30.4% (n = 101) of the patients had abnormal levels (i.e., above the upper limit of the reference range) of CA 15-3, CA 125, and [beta]-2 microglobulin, respectively.
Huang, "Tuberculous peritonitis in a haemodialysis patient with elevated serum CA 125 and hypercalcaemia," International Journal of Clinical Practice, vol.
CA 125 is raised in ovarian cancer as well as other situations such as during menstruation, pregnancy, benign ovarian cysts and endometriosis.
Results of previous studies have suggested that HE4 has diagnostic sensitivity similar to that of CA 125, but an increased diagnostic specificity in patients with gynecologic malignancies compared with those with a benign gynecologic disease (7, 8, 22).
Tumor Marker Control is intended for use as an assayed control serum to monitor the precision of laboratory-testing procedures for the analysis of AFP, CA 15-3, CA 19-9, CA 125, CEA, Ferritin, HE4, PSA, and Free PSA.
The test is undergoing further trials in 500 patients, but until its effectiveness is proven with a larger study, the current gold standard for ovarian cancer screening remains the CA 125 blood test and transvaginal sonogram.