The breadth of repertoire yet beautiful specificity of the antibody response is the key to its physiological efficacy in vivo; it also underpins the attractiveness of antibodies as laboratory and clinical reagents. One aspect of the body’s reaction to invasion by a microorganism is the activation and clonal expansion of antigen-reactive B lymphocytes. Once these have matured into plasma cells, each clone of cells will secrete its own unique specificity of antibody-thus, the invading pathogen will be met by a barrage of antibody molecules capable of binding to many different sites on its surface. Such a polyclonal response, whose range of specificities and affinities can shift with time, is ideal for combatting infection, and indeed for certain laboratory applications (such as secondary reagents for immunoassay); however, in many experimental and clinical situations the ability to have an unlimited supply of a single antibody that is clearly defined and of reproducible specificity and affinity is of greater value. To produce such a reagent it is necessary to isolate and culture a single clone of B lymphocytes secreting antibody of the appropriate characteristics-that is, to produce a monoclonal antibody (mAb).