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 ?
Primary lymphoma of ovary needs confirmation by IHC which is positive for leucocyte common antigen and CD20.
In this example, immunohistochemical studies demonstrated that the RS-like cells were positive for CD30 and CD15 and negative for CD45 (leukocyte common antigen) and B-cell markers including CD20, PAX-5, CD10, and BCL-6.
The possibility of this tumor representing a spindle cell lymphoma was excluded owing to negative staining for CD45 (leukocyte common antigen) and CD5.
Notably, conflicting data were reported for several parameters in the literature, even for a common antigen. Variation among reported effects may be attributable to (1) the tissue type used, as differences in procurement strategy, specimen density, and cell type may translate to differences in susceptibility; (2) the utilization of different antigen retrieval methods, which may introduce additional or mitigate existing effects associated with other preanalytical variables; (3) antibody specificity; (4) differences among the method of quantification; (5) the type and strength of the applied statistical test; and (6) incomplete methodology.
PAX5 and TdT are also commonly seen, while expression of leukocyte common antigen (CD45), CD34, and CD20 may show any pattern of expression from bright to negative (see Figure 3, c for a representative example).
The pleomorphic, large, epithelioid, and spindle-shaped neoplastic cells of both specimens showed a diffuse cytoplasmic positivity for vimentin and membranous expression of leukocyte common antigen and UCHL-1 (Figure 6).
There was no reaction with antibodies directed against S100 protein and leukocyte common antigen, while staining with the monocyte-macrophage marker CD68 was globally strongly positive in both the stromal and giant cells.
(4) In our case, we observed membranous positivity for leukocyte common antigen in the histiocytes that infiltrated the salivary gland, which in our opinion can be regarded as additional evidence that these cells originate from bone marrow-derived monocytes or macrophages.
They were negative for epithelial membrane antigen, cytokeratin (high and low molecular weight), CK20, Ber-EP4, breast-2 antibodies, estrogen receptor, progesterone receptor, muscle-specific actin, smooth muscle actin, desmin, HMB-45, S100, neuron-specific enolase, chromogranin, synaptophysin, CD34, leukocyte common antigen, CD3, CD20, CD57, CD68, and factor 13a.
The sections were negative for cytokeratin 20, leukocyte common antigen, CD20, and CD3.
Immunohistochemically, the cells were focally positive for vimentin; diffusely and strongly positive for immunoglobulin (Ig) A (Figure 3, A) and [Kappa] light chains (Figure 3, B); and negative for IgG, IgM, [Lambda] light chains, leukocyte common antigen, CD20, cytokeratin, S100, HMB45, and smooth muscle antigen.
Gastric large cell lymphoma expressing cytokeratin but no leukocyte common antigen: a diagnostic dilemma.

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