Many breakthroughs in biomedical science have been achieved by the use of polyclonal and monoclonal antibodies. One of the earliest, and probably best-known examples, is the discovery by Behring and Kitasato in the 1890s1 of the therapeutic effects of diphtheria antiserum. The history of monoclonal antibodies, however, is much younger and dates to the pioneering work of Köhler and Milstein in 1975.2 Nowadays, polyclonal antibodies (Pabs) and monoclonal antibodies (Mabs) are indispensable tools in the laboratory. They are used for immunoassays (e.g., as a diagnostic tool), for affinity chromatography, as immunomarkers (e.g., in pathology), and in basic research (e.g., to discover new proteins and to characterize complex antigenic structures). Furthermore, they are of crucial value in the clinic. Although vaccines have replaced most therapeutic polyclonal antisera, some, such as rabies antiserum and snake antivenom are still important in third world countries. Mabs are increasingly being used as carriers in drug-targeted therapy.