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

An ordinary IgG has two matching immunoglobulin heavy chains and two matching immunoglobulin light chains, abbreviated $H_2{L}_2$. Its gamma-type immunoglobulin heavy chains contribute both to recognition and to interactions with immune partners.^[ref]

A useful way to read this picture is to follow a single polypeptide chain continuously from its N-terminus to its C-terminus, then examine the crosslinks separately. The same immunoglobulin heavy chain contributes to a Fab fragment and to the Fc fragment; these named fragments are not pre-existing separate chains. Conversely, one Fab fragment contains material from two different chains. The protein primary structure and the fragment inventory therefore answer different questions about the same protein.

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.

The nitrate ion, ${\mathrm{NO}}_3^{-}$, is an anion with one nitrogen connected to three oxygen chemical atoms and net electric charge $-1$ in units of elementary charge. Its 24 valence electrons admit three equivalent octet resonance structures; see Lewis structure for their construction and chemical resonance for their interpretation. The N–O connections are covalent bonds within the ion. See 7.2 Covalent Bonding.

A manganese-containing anion with net electric charge $-e$ and manganese oxidation state $+7$; $e$ is the positive elementary charge.

Chloride is the anion $\mathrm{C}{\mathrm{l}}^{-}$, with net electric charge $-e$. In the glycine titration model, it supplies the negative background electric charge introduced with hydrochloric acid.

Online text: openstax.org/books/anatomy-and-physiology-2e/pages/3-6-cellular-differentiation