You may have noticed that in each of the domains Fab and Fc the polypeptide chains are organised in two regions (again called domains) with globular structure and very similar.
To study it better, let's switch to a schematic rendering: trace cartoon planks
These domains define a type of folding, or structural motif: the immunoglobulin fold, which is characteristic of immunoglobulins and present as well in other, related, proteins.
While studying haemoglobin, we observed its secondary structure to be predominantly alpha helix; note how there is barely any of that in immunoglobulin, but almost everything is beta sheet. To see it better: colour by type of secondary structure (alpha beta none).
If you want to study in detail how is the structure of each domain in this immunoglobulin fold, please visit the extension; else, jump to the next section.