Antibodies (Ab) are molecules with dual functions: on the one hand, they bind to antigens (Ag) through their variable regions (Fab, "Fragment antigen binding"), located at one of their extremities; on the other hand, they recruit cells of the immune system, via the other extremity, the constant region or Fc region, which results in a selective destruction of cells that have the corresponding Ag. The capacity of recognition of Ag by Ab is unique. Ag generally have several different epitopes that all are binding sites for Ab. Ab may be classified according to their ability to recognize one single epitope or several epitopes. They are called monoclonal Ab (mAb) or polyclonal Ab, respectively. MAbs recognize the same epitope because they are issued from one single line of plasmocytes, originating from one single cell. It is the reason why they are so selective. MAbs exert complex, but unique mechanisms of action: they inhibit or activate signal transduction, and they specifically drive the immune system against target cells, such as tumoral cells. After their extensive use in fundamental and in applied research as wells as diagnostic tools, mAbs are now largely exploited in therapeutics.