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. 2020 Dec 22;7:622473. doi: 10.3389/fcvm.2020.622473

Figure 4.

Figure 4

Schematic representation of an antibody structure and different antibody fragment formats. (A) The left panel shows the cartoon and biological surface representation of the full-size crystal structure of Pembrolizumab, a human anti-PD1 immunoglobulin G4 (IgG4) antibody [PDB ID 5DK3 (226)]. A conventional antibody is a Y-shaped heterotetrameric glycoprotein consisting of two identical heavy (cyan) and two identical light chains (green). The heavy chain comprises one variable domain (VH) and three constant domains (CH1, CH2, and CH3), whereas the light chain comprises one variable (VL) and one constant (CL) domain. The panel on the right shows a simplified schematic representation of a conventional antibody and a heavy-chain-only antibody (HCAb). Each arm of the conventional antibody represents the antigen-binding fragment (Fab) that comprises the constant region (CH1 and CL domains) and the variable fragment (Fv) containing VL and VH. The stem of the antibody comprises two copies of the CH2-CH3 domains that form the crystallizable fragment (Fc). The HCAb comprises two heavy chains, each combining one variable VHH domain, referred to as nanobody, and two constant domains (CH2 and CH3). (B) Schematic representation of a selection of antibody fragments, including Fab, diabody, single-chain variable fragment (scFv), and nanobody. Figure adapted from Rodrigo et al. (227).