Editing Antigen (section)

==Terminology==
* [[Epitope]] – the distinct surface features of an, its ''antigenic determinant''.<br />Antigenic molecules, normally "large" biological polymers, usually present surface features that can act as points of interaction for specific antibodies. Any such feature constitutes an epitope. Most antigens have the potential to be bound by multiple antibodies, each of which is specific to one of the antigen's epitopes. Using the "lock and key" metaphor, the antigen can be seen as a string of keys (epitopes) each of which matches a different lock (antibody). Different antibody [[idiotypes]], each have distinctly formed [[complementarity-determining region]]s.
* [[Allergen]] – A substance capable of causing an [[allergic reaction]]. The (detrimental) reaction may result after exposure via ingestion, inhalation, injection, or contact with skin.
* [[Superantigen]] – A class of antigens that cause non-specific activation of T-cells, resulting in polyclonal T-cell activation and massive [[cytokine]] release.
* [[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]].
* [[Immunoglobulin]]-binding protein – Proteins such as [[protein A]], [[protein G]], and [[protein L]] that are capable of binding to antibodies at positions outside of the antigen-binding site. While antigens are the "target" of antibodies, immunoglobulin-binding proteins "attack" antibodies.
* T-dependent antigen – Antigens that require the assistance of T cells to induce the formation of specific antibodies.
* T-independent antigen – Antigens that stimulate B cells directly.
* Immunodominant antigens – Antigens that dominate (over all others from a [[pathogen]]) in their ability to produce an immune response. T cell responses typically are directed against a relatively few immunodominant epitopes, although in some cases (e.g., infection with the [[malaria]] pathogen ''[[Plasmodium|Plasmodium spp.]]'') it is dispersed over a relatively large number of parasite antigens.<ref name="pmid12886016">{{cite journal | vauthors = Doolan DL, Southwood S, Freilich DA, Sidney J, Graber NL, Shatney L, Bebris L, Florens L, Dobano C, Witney AA, Appella E, Hoffman SL, Yates JR, Carucci DJ, Sette A | title = Identification of Plasmodium falciparum antigens by antigenic analysis of genomic and proteomic data | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 100 | issue = 17 | pages = 9952–57 | date = August 2003 | pmid = 12886016 | pmc = 187898 | doi = 10.1073/pnas.1633254100 | bibcode = 2003PNAS..100.9952D | doi-access = free }}</ref>

[[Antigen-presenting cell]]s present antigens in the form of peptides on [[histocompatibility molecule]]s. The T cells selectively recognize the antigens; depending on the antigen and the type of the histocompatibility molecule, different types of T cells will be activated. For T-cell receptor (TCR) recognition, the peptide must be processed into small fragments inside the cell and presented by a [[major histocompatibility complex]] (MHC).<ref>Parham, Peter. (2009). ''The Immune System'', 3rd Edition, p. G:2, Garland Science, Taylor and Francis Group, LLC.</ref> The antigen cannot elicit the immune response without the help of an [[adjuvant|immunologic adjuvant]].<ref name="ReferenceA">{{cite journal | vauthors = Gavin AL, Hoebe K, Duong B, Ota T, Martin C, Beutler B, Nemazee D | title = Adjuvant-enhanced antibody responses in the absence of toll-like receptor signaling | journal = Science | volume = 314 | issue = 5807 | pages = 1936–1938 | date = December 2006 | pmid = 17185603 | pmc = 1868398 | doi = 10.1126/science.1135299 | bibcode = 2006Sci...314.1936G }}</ref> Similarly, the adjuvant component of vaccines plays an essential role in the activation of the innate immune system.<ref>{{cite journal | vauthors = Janeway CA | title = Pillars article: approaching the asymptote? Evolution and revolution in immunology. Cold spring harb symp quant biol. 1989. 54: 1-13 | journal = Journal of Immunology | volume = 191 | issue = 9 | pages = 4475–4487 | date = November 2013 | pmid = 24141854 }}</ref><ref>{{cite journal | vauthors = Gayed PM | title = Toward a modern synthesis of immunity: Charles A. Janeway Jr. and the immunologist's dirty little secret | journal = The Yale Journal of Biology and Medicine | volume = 84 | issue = 2 | pages = 131–138 | date = June 2011 | pmid = 21698045 | pmc = 3117407 }}</ref>

An immunogen is an antigen substance (or [[adduct]]) that is able to trigger a humoral (innate) or cell-mediated immune response.<ref>Parham, Peter. (2009). ''The Immune System'', 3rd Edition, p. G:11, Garland Science, Taylor and Francis Group, LLC.</ref> It first initiates an innate immune response, which then causes the activation of the adaptive immune response. An antigen binds the highly variable immunoreceptor products (B-cell receptor or T-cell receptor) once these have been generated. Immunogens are those antigens, termed [[immunogenic]], capable of inducing an immune response.<ref>{{cite book|title=Kuby Immunology|publisher=Macmillan|year=2006|isbn=978-1-4292-0211-4|edition= 6th|pages=77}}</ref>

At the molecular level, an antigen can be characterized by its ability to bind to an antibody's [[paratope]]s. Different antibodies have the potential to discriminate among specific epitopes present on the antigen surface. A [[hapten]] is a small molecule that can only induce an immune response when attached to a larger carrier molecule, such as a [[protein]]. Antigens can be proteins, polysaccharides, [[lipid]]s, nucleic acids or other biomolecules.<ref name=Abbas/> This includes parts (coats, capsules, cell walls, flagella, fimbriae, and toxins) of [[bacteria]], [[virus]]es, and other [[microorganism]]s. Non-microbial non-self antigens can include pollen, egg white, and proteins from transplanted tissues and organs or on the surface of transfused blood cells.