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Antibody affinity concerns one paratope binding one epitope. A Fab fragment supplies a single-site comparison. ^[ref] In the dilute chemical equilibrium model below, smaller dissociation constant $K_d$ means larger antibody equilibrium occupancy at the same free antigen concentration. State the antigen, temperature and buffer solution: a dissociation constant is not a context-free score. It does not alone specify how fast attachment or detachment occurs.

Avidity is the combined strength of multivalent attachment. An intact IgG can make two contacts when accessible epitopes have suitable spacing. ^[ref] If one contact breaks while the other remains, the detached arm stays nearby and can bind again before the whole antibody escapes. That geometric advantage does not require changing either paratope's intrinsic antibody affinity. Two arms binding two separate freely moving antigens do not automatically provide the same tethering advantage. Epitope spacing, accessibility and surface density matter; avidity is not obtained by adding two dissociation constants.

A monoclonal antibody preparation derives from one selected antibody-producing biological cell and its descendants, or reproduces their specified antibody sequences. Monoclonal describes the origin and composition of the preparation; it does not guarantee absence of cross-reactivity or establish antibody affinity. ^[ref]

Antibody specificity describes preferential recognition of selected epitopes. It is established relative to tested alternatives and assay conditions, rather than inferred from a reagent's name. Western blot antibody specificity controls distinguish recognition of the proposed protein from retained label caused by other material.

In antibody binding, an antigen is material bearing a recognized epitope. An antigen need not be a protein; polysaccharides can also carry epitopes. One antigen may present several different epitopes, so recognizing one feature does not imply recognizing every part of the target. ^[ref]

A paratope is the recognizing surface on an antibody; the epitope is the recognized feature on its antigen. Contacts depend on spatial arrangement and chemistry, including hydrogen bonds and Van der Waals forces. Thus a paratope is not simply a slot for a protein primary structure string. A folded protein can bring distant amino acid residues into one epitope. ^[ref]

A Fab fragment contains one antibody binding site, allowing comparison with the two-site intact IgG in the antibody surface plasmon resonance experiment.

An antibody constant region is a chain portion outside the antibody variable region. Constant does not mean identical across every antibody class.

The paired antibody variable regions of an immunoglobulin heavy chain and an immunoglobulin light chain contribute to an antibody binding site. Variation refers to differences between antibody sequences, not to a tip continually changing identity while binding.

An immunoglobulin light chain accompanies an arm of the illustrated IgG, contributing to its paratope.

An immunoglobulin heavy chain extends through an arm and the stem of the illustrated IgG.

Immunoglobulin G is an antibody class whose conventional four-chain architecture supplies the example above.

An epitope is the molecular feature recognized by an antibody. Protein denaturation can expose an epitope or destroy the spatial arrangement required for its recognition; an antibody that works on a folded protein need not work in SDS-PAGE-based Western blot antibody detection.

A secondary antibody recognizes another antibody, rather than the proposed target protein directly. In Western blot antibody detection, its attached label makes retained primary antibodies visible. A secondary antibody must recognize the kind of primary antibody used.

In an indirect Western blot, the primary antibody binds an epitope on the proposed target protein. Its binding must be tested in the actual sample and assay conditions through Western blot antibody specificity controls.