Fig. 4

Structural considerations of accessible and inaccessible epitopes in a sandwich immunoassay. (A) Two antibodies can simultaneously bind to the same antigen by interacting with their respective linear (blue) or conformational (red) epitope at the surface of the protein structure. (B) The antibody cannot recognize its epitope (green) as it is buried inside the protein structure, binding to the antigen is unsuccessful. (C) Two epitopes are close together within the native protein fold and one antibody cannot bind its epitope (teal) as it is blocked by the already bound antibody (blue); simultaneous binding to the antigen is unsuccessful