Although poly- and monoclonal antibodies are successfully applied in research, an expected clinical breakthrough of these reagents so far has not occurred. This can mainly be explained by the animal origin of antibodies, which may lead to a deleterious immune response upon therapeutic use in humans. Moreover, it has been technically demanding to alter the desired affinity, format and effector functions of existing antibodies. Currently, antibody phage-display technology, through construction of large and highly diverse antibody libraries, completely by-passing the immune system, enables the isolation of human antibodies, which can be engineered for every desired application.