antigen: Definition and Much More from Answers.com
- ️Wed Jul 01 2015
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.
antigenic an'ti·gen'ic (-jĕn'ĭk) adj.
antigenically an'ti·gen'i·cal·ly adv.
antigenicity an'ti·ge·nic'i·ty (-jə-nĭs'ĭ-tē) n.
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 also 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 also 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 also 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 also 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 also Heterophile antigen; Hypersensitivity; Immunology.
Any compound that is foreign to the body (e.g. bacterial, food, or pollen protein, or some complex carbohydrates) which, when introduced into the circulation, stimulates the formation of an antibody. See also adverse reactions to foods.
(an′tijen)
n
A substance, usually a protein, that elicits the formation of antibodies that react with it when introduced parenterally into an individual or species to which it is foreign.
An antigen is a substance that is capable of inducing a specific immune response in the host into which it is introduced. The immune response is mediated via an immunoglobulin (protein) molecule, called an antibody, which is formed by B-lymphocytes and T-helper cells that are the basic ingredients of the host's immune system. "Antibody" is the generic name for any immunoglobulin thus produced, no matter how this occurs. Humans can produce many specific antibodies. This may be an active process by a healthy host in response to the challenge of exposure to a foreign antigen transmitted via the placenta or in maternal milk from mother to offspring, or it may be artificially induced by immunization with live attenuated organisms, killed organisms, or a protein derivative.
An antigen is an organic compound—a protein, polysaccharide or glycolipid. Sometimes it is an entire organ or tissue that has been transplanted into the host, which rejects it and attempts to destroy it. An antibody has the capacity to bind specifically to the (foreign) antigen and thereby neutralize it so it can be destroyed by the host's phagocytes.
The antibody is the basic ingredient of the host's defenses against infection. By measuring the concentration of specific antibodies in individuals and populations it is possible to determine levels of susceptibility and resistance to infection by specific pathogens. At the population level, this is called "sero-epidemiology."
An antigen is produced by living organisms, which evolve over generations and can therefore undergo subtle changes in protein composition, a process known as "antigen drift." A more sudden evolutionary change can lead to an abrupt change in protein structure, known as "antigen shift." Antigen drift renders an antibody less effective, and antigen shift makes an antibody ineffective in combating an antigen to which the host was immunized by exposure to the previous form of the antigen. Antigen drift and antigen shift account for recurrences of infection with viruses, such as those of the common cold and influenza.
(SEE ALSO: Epidemiology; Immunizations)
— JOHN M. LAST
Foreign substance in the body that induces an immune response. The antigen stimulates lymphocytes to produce antibodies or to attack the antigen directly (see antibody;immunity). Virtually any large foreign molecule can act as an antigen, including those of bacteria, viruses, parasites, foods, venoms, blood components, and cells and tissues of various species, including other humans. Sites on the antigen's surface fit and bind to receptor molecules on the lymphocytes' surface, stimulating the latter to multiply and initiate an immune response that neutralizes or destroys the antigen.
For more information on antigen, visit Britannica.com.
A substance (usually a protein or carbohydrate) that is identified by cells in the immune system as being foreign to the body, and which induces an immune response.
The capacity to stimulate the production of antibodies or cell-mediated immune responses.
An antigen or immunogen is a molecule that stimulates an immune response.
The word originated from the notion that they can stimulate antibody generation. We now know that the immune system
does not only consist of antibodies. The modern definition encompasses all substances that can
be recognized by the adaptive immune system.
Antigens are usually proteins or polysaccharides. This includes parts (coats, capsules, cell walls, flagella, fimbrae, and toxins) of bacteria, viruses, and other microorganisms. Lipids and nucleic acids are antigenic only when combined with proteins and polysaccharides. Non-microbial exogenous (non-self) antigens can include pollen, egg white, and proteins from transplanted tissues and organs or on the surface of transfused blood cells.
- Tolerogen - An antigen that invokes a specific immune non-responsiveness due to its molecular form. If its molecular form is changed, a tolerogen can become an immunogen.
- Allergen - An allergen is a substance that causes the allergic reaction. The (detrimental) reaction may result after exposure via ingestion, inhalation, injection or contact with skin.
Cells present their antigens to the immune system via a histocompatibility molecule. Depending on the antigen presented and the type of the histocompatibility molecule, several types of immune cells can become activated.
Origin of antigens
Antigens can be classified in order of their origins.
Exogenous antigens
Exogenous antigens are antigens that have entered the body from the outside, for example by inhalation, ingestion, or injection. By endocytosis or phagocytosis, these antigens are taken into the antigen-presenting cells (APCs) and processed into fragments. APCs then present the fragments to T helper cells (CD4+) by the use of class II histocompatibility molecules on their surface. Some T cells are specific for the peptide:MHC complex. They become activated and start to secrete cytokines. Cytokines are substances that can activate cytotoxic T lymphocytes (CTL), antibody-secreting B cells, macrophages and other particles.
Endogenous antigens
Endogenous antigens are antigens that have been generated within the cell, as a result of normal cell metabolism, or because of viral or intracellular bacterial infection. The fragments are then presented on the cell surface in the complex with MHC class I molecules. If activated cytotoxic CD8+ T cells recognize them, the T cells begin to secrete different toxins that cause the lysis or apoptosis of the infected cell. In order to keep the cytotoxic cells from killing cells just for presenting self-proteins, self-reactive T cells are deleted from the repertoire as a result of tolerance (also known as negative selection which occurs in the thymus).
Autoantigens
An autoantigen is usually a normal protein or complex of proteins (and sometimes DNA or RNA) that is recognized by the immune system of patients suffering from a specific autoimmune disease. These antigens should under normal conditions not be the target of the immune system, but due to mainly genetic and environmental factors the normal immunological tolerance for such an antigen has been lost in these patients.
Tumor antigens
Tumor antigens or Neoantigens are those antigens that are presented by MHC I or MHC II molecules on the surface of tumor cells. These antigens can sometimes be presented by tumor cells and never by the normal ones. In this case, they are called tumor-specific antigens (TSAs) and typically result from a tumor specific mutation. More common are antigens that are presented by tumor cells and normal cells, and they are called tumor-associated antigens (TAAs). Cytotoxic T lymphocytes that recognized these antigens may be able to destroy the tumor cells before they proliferate or metastasize.
Tumor antigens can also be on the surface of the tumor in the form of, for example, a mutated receptor, in which case they will be recognized by B cells.
See also
External links
- Antigen Retrieval Protocol
- Immunology
- National Library of Medicine/Medline (National Insititute of Health) website
| Immune system / Immunology | |
|---|---|
| Systems | Adaptive immune system vs. Innate immune system • Humoral immune system vs. Cellular immune system • Complement system (Anaphylatoxins) |
| Antibodies and antigens | Antibody (Monoclonal antibodies, Polyclonal antibodies, Autoantibody) • Allotype • Isotype • Idiotype • Antigen (Superantigen) |
| Immune cells | White blood cells (T cell, B cell, NK cell, Mast cell, Basophil, Eosinophil) • Phagocyte (Neutrophil, Macrophage, Dendritic cell) • Antigen-presenting cell • Reticuloendothelial system |
| Immunity vs. tolerance | Immunity • Autoimmunity • Allergy • Tolerance (Central) • Immunodeficiency |
| Immunogenetics | Somatic hypermutation • V(D)J recombination • Immunoglobulin class switching • MHC / HLA |
| Other | Cytokines • Inflammation • Opsonin |
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
Nederlands (Dutch)
antigen (stof die afweersysteem activeert)
Français (French)
n. - antigène
Deutsch (German)
n. - (med.) Antigen
Ελληνική (Greek)
n. - (χημ.) αντιγόνο
Português (Portuguese)
n. - antígeno (m) (Med.)
Español (Spanish)
n. - antígeno
Svenska (Swedish)
n. - antigen
中文（简体） (Chinese (Simplified))
抗原
中文（繁體） (Chinese (Traditional))
n. - 抗原
العربيه (Arabic)
(الاسم) مولد مضاد : ماده ينتج عن حقنهافي الجسم اجسام مضاده
עברית (Hebrew)
n. - חומר זר (למשל רעל) המעורר תגובה חיסונית בגוף, מייצר נוגדנים, אנטיגן
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