= Antibody
{wiki}
An <antibody> is a <protein> that recognizes particular molecular targets through binding. <Antibodies> participate in <immunity>; laboratory assays such as <Western blots> exploit their selective binding to detect chosen <proteins>.
An <IgG> gives a concrete architecture: two <immunoglobulin heavy chains> and two <immunoglobulin light chains>, joined by <disulfide bonds>. Each arm ends in a <paratope> assembled from an <antibody variable region> of each kind of chain. The two <paratopes> recognize the same <epitope> in this conventional <IgG>; the flexible hinge allows different arm separations. The <antibody constant regions> form the remaining portions. This is an illustrative <IgG>, rather than the architecture of every <antibody> format. <Immunobiology: The Immune System in Health and Disease, 5th edition by Charles A. Janeway, Jr. and others/The structure of a typical antibody molecule>[Immunobiology, 5th edition, §§3-1, 3-2 and 3-4, Figures 3.2 and 3.4: four chains, paired sites and hinge flexibility]{ref}
\Image[/antibody-igg-chains-and-bivalent-binding.png]
{title=<IgG> chains, <paratopes> and two-arm binding}
{description=Original schematic of a conventional <IgG>. Color distinguishes <immunoglobulin heavy chains> from <immunoglobulin light chains>; darker tips mark <antibody variable regions>. Each outlined <paratope> includes both chain tips. The second panel contrasts one contact with two contacts to appropriately spaced <epitopes> on an imagined surface. Shapes, spacing and chain paths are illustrative, with no structural coordinates or physical scale.}
Three questions must be kept separate. <Antibody specificity> asks which <antigens> are recognized. <Antibody affinity> describes one <paratope>–<epitope> interaction. <Avidity> describes the combined attachment when several contacts can cooperate. A selective <antibody> can have modest <antibody affinity>, and a <monoclonal antibody> can still show <cross-reactivity>. For an assay, the practical question is whether the observed readout identifies the intended <antigen> under the tested conditions; <Western blot antibody specificity controls> make that question concrete.
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