Paratope
{{short description|Part of an antibody which binds to an antigen}}
[[File:Immunoglobulin basic unit.svg|thumb|
An antibody with a circled region depicting where the paratope is found.
1. Antigen-binding fragment (Fab)
2. Antibody crystallizable region (Fc)
3. Heavy chains
4. Light chains
5. Variable region of the antibody. The paratope is the key-shaped section that makes direct contact with the antigen.{{cite book | vauthors = Lefranc MP |chapter=Paratope|date=2013 | title = Encyclopedia of Systems Biology|pages=1632–1633|place=New York, NY|publisher=Springer|language=en|doi=10.1007/978-1-4419-9863-7_673 |isbn=978-1-4419-9863-7 | veditors = Dubitzky W, Wolkenhauer O, Cho KH, Yokota H }}
6. Hinge regions
]]
In immunology, a paratope, also known as an antigen-binding site, is the part of an antibody which recognizes and binds to an antigen.{{Cite book | vauthors = Punt J, Stranford SA, Jones PP, Owen JA |title=Kuby immunology |date=2019 |isbn=978-1-4641-8978-4 |edition=Eighth |location=New York |oclc=1002672752 }} It is a small region at the tip of the antibody's antigen-binding fragment and contains parts of the antibody's heavy and light chains. Each paratope is made up of six complementarity-determining regions - three from each of the light and heavy chains - that extend from a fold of anti-parallel beta sheets. Each arm of the Y-shaped antibody has an identical paratope at the end.
Paratopes make up the parts of the B-cell receptor that bind to and make contact with the epitope of an antigen. All the B-cell receptors on any one individual B cell have identical paratopes. The uniqueness of a paratope allows it to bind to only one epitope with high affinity and as a result, each B cell can only respond to one epitope. The paratopes on B-cell receptors binding to their specific epitope is a critical step in the adaptive immune response.
Design of paratopes between species
The design and structure of paratopes can differ greatly between different species. In jawed-vertebrates, V(D)J recombination can result in billions of different paratopes.{{cite journal | vauthors = de los Rios M, Criscitiello MF, Smider VV | title = Structural and genetic diversity in antibody repertoires from diverse species | journal = Current Opinion in Structural Biology | volume = 33 | pages = 27–41 | date = August 2015 | pmid = 26188469 | pmc = 7039331 | doi = 10.1016/j.sbi.2015.06.002 }}{{cite journal | vauthors = Litman GW, Rast JP, Fugmann SD | title = The origins of vertebrate adaptive immunity | journal = Nature Reviews. Immunology | volume = 10 | issue = 8 | pages = 543–53 | date = August 2010 | pmid = 20651744 | pmc = 2919748 | doi = 10.1038/nri2807 | url = }} The number of paratopes, however, is limited by the composition of the V, D, and J genes and the structure of the antibody. Thus, many different species have developed ways to bypass this restriction and increase the diversity of possible paratopes.
In cows, an extra-long complementarity-determining region is considered to have an essential role in diversifying paratopes.{{cite journal | vauthors = Wang F, Ekiert DC, Ahmad I, Yu W, Zhang Y, Bazirgan O, Torkamani A, Raudsepp T, Mwangi W, Criscitiello MF, Wilson IA, Schultz PG, Smider VV | display-authors = 6 | title = Reshaping antibody diversity | journal = Cell | volume = 153 | issue = 6 | pages = 1379–93 | date = June 2013 | pmid = 23746848 | pmc = 4007204 | doi = 10.1016/j.cell.2013.04.049 }} Additionally, both chickens and rabbits use gene conversion to increase the number of paratopes that are possible.