The antibody's HV region forms an opening to surround the antigen's protruding Gln 121 (green). Hydrogen bonds (yellow) stabilize the antibody-antigen interaction. In addition to hydrogen bonds, other weak interactions such as van der Waals forces, hydrophobic interactions and electrostatic forces improve the binding specificity between antibody and antigen. These interactions occur over large and sometimes discontinuous regions of the molecules, improving binding affinity.