The mechanism of action of monoclonal antibodies differs by the nature of the intervention for which they are intended. There are many mechanisms of action. One of the possible mechanisms of action is blocking an enzyme. Also, another active substance, such as a radiopharmaceutical or a chemotherapeutic agent, may be bound to the monoclonal antibody. In this case the monoclonal antibody indeed does serve as a “means of transport” and on the basis of a specific bond in a certain tissue the active substance itself gets directly to the site of action, in the concerned case into the tumour cell.