Researchers can make monoclonal antibodies by immunizing a mouse with a molecule (or with a microorganism). The resulting antibody-mediated response produces a set of antibodies that recognize different parts of the molecule. The mouse's B cells are then harvested from its spleen and fused with cancerous B cells from a myeloma cell line. The resulting hybrid myeloma ("hybridoma") cells are cloned: Individual cells are grown in tissue culture as separate cell lines. Each cell line produces and secretes antibodies that recognize one part of the immunizing molecule. These antibodies are called monoclonal antibodies, and they can be purified and used for research or other purposes. Monoclonal antibodies are effective for passive immunization, but only in the immediate term. Antibodies produced by one's own immune system can last up to about six months in the bloodstream, but monoclonals delivered in passive immunization often last for less than a week. Why the difference?