Antigen-antibody binding occurs at
General features of the antigen-antibody reaction

The reaction is specific, an antigen combines with only its homologous antibody and vice versa, the specificity however is not absolute.
Antigen-Antibody reaction requires the presence of electrolytes at a specific pH and temperature
Bonds involved in Antigen-Antibody reaction are - Van der Waal's bond, Ionic bonds, and Hydrogen bonding
Covalent bonds are not involved 
There is no denaturation of the antigen or antibody during the reaction
Entire molecule of the antigen and antibody reacts
The combination occurs at the surface. Therefore it is surface antigens that are immunologically relevant
The combination is firm and reversible
Both antigen and antibody participate in the formation of agglutinates or precipitates.
Center
Inside molecule
Anywhere in structure
Surface