The antibody molecule is a therapeutic agent, designed by nature to bind to a wide range of antigen molecules and to trigger effector functions, such as complement lysis and cell-mediated killing. The genes encoding antibodies can be manipulated in vitro , allowing the binding sites for antigen and effector molecules to be dissected, and new properties to be engineered. The future for the application of engineered antibodies in medicine is reviewed in the context of the past century.