Antigen
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{{split-apart|Antigen|Immunogen}}
An '''antigen''' (from ''antibody-generating'') or '''immunogen'''{{Fact|date=March 2008}} is a substance that prompts the generation of [[antibodies]] and can cause an immune response.<ref>{{cite web
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</ref> The word originated from the notion that they can stimulate '''anti'''body '''gen'''eration. We now know that the immune system does not consist of only [[antibodies]]. The modern definition encompasses all substances that can be recognized by the [[adaptive immune system]].
Antigens are usually [[protein]]s or [[polysaccharide]]s. This includes parts (coats, capsules, cell walls, flagella, fimbrae, and toxins) of [[bacteria]], [[virus]]es, and other [[microorganism]]s. [[Lipid]]s and [[nucleic acid]]s 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''' - A substance 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 cell]]s 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 cell]]s (APCs) and processed into fragments. APCs then present the fragments to [[T helper cells]] ([[CD4]]<sup>+</sup>) by the use of [[MHC class II|class II histocompatibility]] molecules on their surface. Some T cells are specific for the peptide:MHC complex. They become activated and start to secrete [[cytokine]]s. 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 T cell|cytotoxic CD8<sup>+</sup> T cells]] recognize them, the T cells begin to secrete various [[toxin]]s 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]]).
===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==<!-- This section is linked from [[Immune system]] -->
'''Tumor antigens''' or '''Neoantigens''' are those antigens that are presented by [[MHC I]] or [[MHC II]] molecules on the surface of [[tumor cell]]s. 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, in general, 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 recognize 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]].
==Nativity==
A ''native antigen'' is an antigen which isn't yet processed by an APC to smaller parts. [[T cells]] cannot bind native antigens, but require that they are processed by APCs, while [[B cells]] can be activated by native ones.
==See also==
* [[Epitope]]
* [[Linear epitope]]
* [[Conformational epitope]]
* [[Original antigenic sin]]
* [[Polyclonal B cell response]]
==References==
{{reflist}}
==External links==
*[http://www.histochem.net/histochemistry%20protocol%20antigen%20retrieval.htm Antigen Retrieval Protocol]
*[http://www.theimmunology.com Immunology]
*[http://www.nlm.nih.gov/medlineplus/ency/article/002224.htm National Library of Medicine/Medline (National Insititute of Health) website]
{{immune_system}}
[[Category:Immune system]]
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