Monoclonal antibodies are identical antibodies produced in a laboratory from a single B cell. Researchers make monoclonal antibodies by injecting a mouse with a target antigen and then fusing B cells from the mouse with another long-lived cell. The resulting hybrid cell becomes a type of antibody factory, turning out identical copies of antibody molecules specific for the target antigen. Monoclonal antibodies are used to treat cancer, some viral infections, inflammatory diseases, some cardiovascular diseases, and organ transplant rejections.