Editing Epitope (section)

==Function==

=== T cell epitopes ===
[[T cell]] epitopes<ref>{{cite journal | vauthors = Steers NJ, Currier JR, Jobe O, Tovanabutra S, Ratto-Kim S, Marovich MA, Kim JH, Michael NL, Alving CR, Rao M | display-authors = 6 | title = Designing the epitope flanking regions for optimal generation of CTL epitopes | journal = Vaccine | volume = 32 | issue = 28 | pages = 3509–16 | date = June 2014 | pmid = 24795226 | doi = 10.1016/j.vaccine.2014.04.039 }}</ref> are presented on the surface of an [[antigen-presenting cell]], where they are bound to [[major histocompatibility complex]] (MHC) molecules. In humans, [[Antigen-presenting cell#Professional APCs|professional antigen-presenting cell]]s are specialized to present [[MHC class II]] peptides, whereas most nucleated [[somatic cell]]s present MHC class I peptides. T cell epitopes presented by [[MHC class I]] molecules are typically peptides between 8 and 11 amino acids in length, whereas MHC class II molecules present longer peptides, 13–17 amino acids in length,<ref>{{cite book | vauthors = Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P |title=Molecular biology of the cell |date=2002 |publisher=Garland Science |location=New York |isbn=978-0-8153-3218-3 |edition=4th |page=1401 }}</ref> and non-classical MHC molecules also present non-peptidic epitopes such as [[glycolipid]]s.

=== B cell epitopes ===
The part of the antigen that immunoglobulin or antibodies bind to is called a B-cell epitope.<ref name="Sanchez-Trincado et al 2017">{{cite journal |last1=Sanchez-Trincado |first1=Jose L. |last2=Gomez-Perosanz |first2=Marta |last3=Reche |first3=Pedro A. |title=Fundamentals and Methods for T- and B-Cell Epitope Prediction |journal=Journal of Immunology Research |date=2017 |volume=2017 |pages=1–14 |doi=10.1155/2017/2680160 |pmid=29445754 |pmc=5763123 |doi-access=free }}</ref> B cell epitopes can be divided into two groups: conformational or linear.<ref name="Sanchez-Trincado et al 2017"/> B cell epitopes are mainly conformational.<ref>{{cite journal | vauthors = El-Manzalawy Y, Honavar V | title = Recent advances in B-cell epitope prediction methods | journal = Immunome Research | volume = 6 | issue = Suppl 2 | pages = S2 | date = November 2010 | pmid = 21067544 | pmc = 2981878 | doi = 10.1186/1745-7580-6-S2-S2 | doi-access = free }}</ref><ref name=Regenmortel2009>{{cite book |doi=10.1007/978-1-59745-450-6_1 |chapter=What is a B-Cell Epitope? |title=Epitope Mapping Protocols |series=Methods in Molecular Biology |year=2009 |last1=Regenmortel |first1=Marc H.V. |volume=524 |pages=3–20 |pmid=19377933 |isbn=978-1-934115-17-6 }}</ref> There are additional epitope types when the quaternary structure is considered.<ref name=Regenmortel2009/> Epitopes that are masked when protein subunits aggregate are called [[cryptotope]]s.<ref name=Regenmortel2009/> Neotopes are epitopes that are only recognized while in a specific quaternary structure and the residues of the epitope can span multiple protein subunits.<ref name=Regenmortel2009/> Neotopes are not recognized once the subunits dissociate.<ref name=Regenmortel2009/>

===Cross-activity===
Epitopes are sometimes cross-reactive.  This property is exploited by the immune system in regulation by anti-idiotypic antibodies (originally proposed by Nobel laureate [[Niels Kaj Jerne]]). If an antibody binds to an antigen's epitope, the paratope could become the epitope for another antibody that will then bind to it. If this second antibody is of [[IgM]] class, its binding can upregulate the immune response; if the second antibody is of [[IgG]] class, its binding can downregulate the immune response.{{citation needed|date=September 2012}}