Antibodies bind to antigens in a reversable non-covalent manner via hydrogen bonds, ionic bonds, hydrophobic interactions and van der Waal's interactions. Antibodies only react with antigens in solution -- as opposed to TCRs, which react with antigens bound to cell surfaces. These forces operate at short distances, so the antibody CDR (accounting for most of the antigen-antibody interaction) and the antigen epitope (where the CDR binds) must fit together very well. The more precise the fit, the better the interaction.
Antibodies make contact with protein antigens, usually 15-22 amino acids on the antigen contact a similar number on the antibody giving a complementary surface of 650-900 Angstroms. The amino acids comprising the epitope are adjacent in 3D space, but not necessarily in linear sequence.
When exposed to an antigen, B cells producing antibodies reactive to that antigen will begin to produce vast amounts of antibody. Each B cell produces a monoclonal population of antibodies, meaning each antibody binds the same epitope (site) on the same antigen; however, the B cells together produce a polyclonal population of antibodies, consisting of different antibodies binding a different epitope (site) on the same antigen.