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In [[biochemistry]], '''avidity''' refers to the accumulated strength of ''multiple'' affinities of individual [[non-covalent]] binding interactions, such as between a protein receptor and its [[Ligand (biochemistry)|ligand]], and is commonly referred to as functional affinity. Avidity differs  from [[chemical affinity|affinity]], which describes the strength of a ''single'' interaction. However, because individual binding events increase the likelihood of occurrence of other interactions (i.e., increase the local concentration of each binding partner in proximity to the binding site), avidity should not be thought of as the mere sum of its constituent affinities but as the combined effect of all affinities participating in the biomolecular interaction. A particular important aspect relates to the phenomenon of 'avidity entropy'.<ref>{{cite journal | vauthors = Kitov PI, Bundle DR | title = On the nature of the multivalency effect: a thermodynamic model | journal = Journal of the American Chemical Society | volume = 125 | issue = 52 | pages = 16271–84 | date = December 2003 | pmid = 14692768 | doi = 10.1021/ja038223n }}</ref> Biomolecules often form heterogenous complexes or homogeneous [[oligomer]]s and multimers or [[polymer]]s. If clustered proteins form an organized matrix, such as the [[clathrin]]-coat, the interaction is described as a [[matricity]].{{citation needed|date=January 2016}}

==Antibody-antigen interaction==
Avidity is commonly applied to [[antibody]] interactions in which multiple antigen-binding sites simultaneously interact with the target [[antigen]]ic [[epitope]]s, often in multimerized structures. Individually, each binding interaction may be readily broken; however, when many binding interactions are present at the same time, transient unbinding of a single site does not allow the molecule to diffuse away, and binding of that weak interaction is likely to be restored.{{citation needed|date=January 2016}}

Each antibody has at least two antigen-binding sites, therefore antibodies are bivalent to multivalent. Avidity (functional affinity) is the accumulated strength of multiple affinities.<ref name="pmid19409036">{{cite journal | vauthors = Rudnick SI, Adams GP | title = Affinity and avidity in antibody-based tumor targeting | journal = Cancer Biotherapy & Radiopharmaceuticals | volume = 24 | issue = 2 | pages = 155–61 | date = April 2009 | pmid = 19409036 | pmc = 2902227 | doi = 10.1089/cbr.2009.0627 }}</ref> For example, [[IgM]] is said to have low affinity but high avidity because it has 10 weak binding sites for antigen as opposed to the 2 stronger binding sites of [[Immunoglobulin G|IgG]], [[IgE]] and [[IgD]] with higher single binding affinities.{{citation needed|date=January 2016}}

===Affinity===
Binding affinity is a measure of dynamic equilibrium of the ratio of on-rate (k<sub>on</sub>) and off-rate (k<sub>off</sub>) under specific concentrations of reactants. The affinity constant, [[Equilibrium constant|K<sub>a</sub>]], is the inverse of the dissociation constant, [[Dissociation constant|K<sub>d</sub>]]. The strength of complex formation in solution is related to the [[stability constants of complexes]], however in case of large biomolecules, such as [[Receptor (biochemistry)|receptor]]-[[Ligand (biochemistry)|ligand]] pairs, their interaction is also dependent on other structural and thermodynamic properties of reactants plus their orientation and immobilization.{{citation needed|date=January 2016}}

There are several [[methods to investigate protein–protein interactions]] existing with differences in immobilization of each reactant in 2D or 3D orientation. The measured affinities are stored in public databases, such as the [[Ki Database]] and [[BindingDB]].{{citation needed|date=January 2016}} As an example, affinity is the binding strength between the complex structures of the [[epitope]] of antigenic determinant and [[paratope]] of antigen-binding site of an antibody. Participating non-covalent interactions may include [[hydrogen bond]]s, [[ionic bond|electrostatic bond]]s, [[van der Waals force]]s and [[hydrophobic effect]]s.<ref>{{cite book |first1=Charles A |last1=Janeway |first2=Paul |last2=Travers |first3=Mark |last3=Walport |first4=Mark J |last4=Shlomchik | name-list-style = vanc |page=[https://archive.org/details/immunobiology00char/page/104 104] |year=2001 |title=Immunobiology |edition=5th |url=https://archive.org/details/immunobiology00char/page/104 |location=New York |publisher=Garland Science |isbn=0-8153-3642-X |url-access=registration }}{{page needed|date=January 2016}}</ref>

Calculation of binding affinity for bimolecular reaction (1 antibody binding site per 1 antigen):

:<chem>[Ab] + [Ag] <=> [AbAg]</chem>

where [Ab] is the [[antibody]] concentration and [Ag] is the [[antigen]] concentration, either in free ([Ab],[Ag]) or bound ([AbAg]) state.

calculation of association constant (or [[equilibrium constant]]):   
:<math chem>K_a = \frac{k_\ce{on}}{k_\ce{off}} = \frac\ce{[AbAg]}\ce{[Ab][Ag]}</math>

calculation of dissociation constant:  
:<math chem>K_d = \frac{k_\ce{off}}{k_\ce{on}} = \frac\ce{[Ab][Ag]}\ce{[AbAg]}</math>

==Application==
Avidity tests for [[rubella virus]], ''[[Toxoplasma gondii]]'', [[cytomegalovirus]] (CMV), [[varicella zoster virus]], human immunodeficiency virus ([[HIV]]), [[viral hepatitis|hepatitis viruses]], [[Epstein–Barr virus]], and others were developed a few years ago. These tests help to distinguish acute, recurrent or past infection by avidity of marker-specific [[IgG]]. Currently there are two avidity assays in use. These are the well known chaotropic (conventional) assay and the recently developed AVIcomp (avidity competition) assay.<ref name="pmid19129411">{{cite journal | vauthors = Curdt I, Praast G, Sickinger E, Schultess J, Herold I, Braun HB, Bernhardt S, Maine GT, Smith DD, Hsu S, Christ HM, Pucci D, Hausmann M, Herzogenrath J | display-authors = 6 | title = Development of fully automated determination of marker-specific immunoglobulin G (IgG) avidity based on the avidity competition assay format: application for Abbott Architect cytomegalovirus and Toxo IgG Avidity assays | journal = Journal of Clinical Microbiology | volume = 47 | issue = 3 | pages = 603–13 | date = March 2009 | pmid = 19129411 | pmc = 2650902 | doi = 10.1128/JCM.01076-08 }}</ref>

== See also ==
*[[Amino acid residue]]
*[[Epitope]]
*[[Fab region]]
*[[Hapten]]
A number of technologies exist to characterise the avidity of molecular interactions including [[switchSENSE]] and [[surface plasmon resonance]].<ref>{{cite journal | vauthors = Gjelstrup LC, Kaspersen JD, Behrens MA, Pedersen JS, Thiel S, Kingshott P, Oliveira CL, Thielens NM, Vorup-Jensen T | display-authors = 6 | title = The role of nanometer-scaled ligand patterns in polyvalent binding by large mannan-binding lectin oligomers | journal = Journal of Immunology | volume = 188 | issue = 3 | pages = 1292–306 | date = February 2012 | pmid = 22219330 | doi = 10.4049/jimmunol.1103012 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Vorup-Jensen T | title = On the roles of polyvalent binding in immune recognition: perspectives in the nanoscience of immunology and the immune response to nanomedicines | journal = Advanced Drug Delivery Reviews | volume = 64 | issue = 15 | pages = 1759–81 | date = December 2012 | pmid = 22705545 | doi = 10.1016/j.addr.2012.06.003 }}</ref>

== References ==
{{Reflist}}

== Further reading ==
{{refbegin}}
* {{cite book | first1 = Ivan M | last1 = Roitt | first2 = Jonathan | last2 = Brostoff | first3 = David K | last3 = Male | name-list-style = vanc | title = Immunology | edition = 6th | date = 2001 | publisher = Mosby Publishers | page = 72 | isbn = 978-0-7234-3189-3 }}
{{refend}}

== External links ==
* {{MeshName|Antibody+Avidity}}

[[Category:Biophysics]]
[[Category:Protein structure]]