Which one of the combinations of the following statements is true about antibody structure?
- Limited proteolysis of rabbit IgG with the enzyme pepsin generates two antigen-binding regions (Fab) and an Fc fragment.
- Limited proteolysis of rabbit IgG with the enzyme papain generates a single bivalent antigen-binding region F(ab')$_2$ and peptide fragments.
- The Fc fragment of IgG can self-associate and crystallize into a lattice.
- The F(ab')$_2$ fragment of IgG is composed of both light and heavy chains.
- P and Q only
- P and R only
- R and S only
- Q and S only