Here we can observe an Fab fragment from anti-lysozyme IgG bound to its antigen, lysozyme. (heavy chain H, light chain L, lysozyme)
Remember that the variable domain includes the binding site for the antigen. Within each of the variable regions (VH and VL domains) there are three hypervariable regions, where the amino acid variation among the IgGs that bind different antigens is maximal. This is so because within them are the amino acid residues in the antibody that directly contact the antigen. These regions are also called complementarity determining regions (CDR).
atoms in the antibody that contact the antigen atoms in the antigen in the contact zone (antigenic determinant of lysozyme, or epitope recognised by this antibody).