The left shows the structure of IgG2a and the right shows a diagram of the same antibody with the two domains of the antibody labeled. The Fab domain contains the part that recognizes the antigen and the Fc domain contains that part that binds to the Fc receptor on immune cells. Cleaving the antibody at the dotted line produces the two fragments. The Fc domains stay together because of the bonds between the heavy chains. The Fab domains separate into 2 linked fragments each containing the light chain and part of the heavy chain that make the antigen-recognition site. The antibody structure is from PDB:1IGT. Read more:
N. R. Gough, Building Better Antibodies. BioSerendipity (6 November 2018) https://www.bioserendipity.com/building-better-antibodies/.