After a period of euphoria following the report on the first ex vivo production of monoclonal antibodies (Köhler, 1975), the new “magic bullets” as they were qualified by the press, however did not keep theirs promise. Indeed, apart OKT3 a mouse mAb directed against the T lymphocyte CD3 molecule, the first to get US and European approvals in the prevention of organ rejection in 1986, several trials conducted with other mAbs did not show significant results. These poor effects were mainly due to generation of anti-murine Abs by the patients, hindering the efficacy of the treatment. Moreover, rodent IgG Abs injected to patients have half-life of less than 20 hrs compared to several days for human Igs.