Introduction - This script shows the structure of an antibody. Scroll down until the text corresponding to the view is visible. (Based on 1dbb.pdb, J.H.Arevalo J. Mol. Biol. 231 103 (1993)).
This is the antigen-binding domain of one Fab fragment. (Each IgG has 2 Fab fragments). It consists of a heavy chain constant domain, a light chain constant domain, a heavy chain variable domain and a light chain variable domain.
This is the immunoglobulin fold of the constant domain. Note that the immunoglobulin fold is composed of two tightly packed antiparallel β sheets.
This is the immunoglobulin fold of the variable domain. In addition to the β sheet of the immunoglobulin fold, the variable domain contains three loops, which contain the antigen recognition domain.
Here is an antigen bound to the antigen-recognition domain composed of the variable domains of the light and heavy chain. Each antibody has two antigen binding domains, one at the variable end of each Fab region.
This view shows the tight packing of the antigen and the antigen-binding domain.