Monoclonal antibodies are identical antibodies produced by a single clone of B cells that recognize the same epitope on an antigen. They can be produced in a laboratory by fusing B cells that produce a desired antibody with myeloma cells to form hybridomas. Monoclonal antibodies have many diagnostic applications as they can be used to detect specific substances like proteins, pathogens, and tumor markers. They allow for rapid diagnosis of diseases like hepatitis, influenza, and cancer. Monoclonal antibodies are also used in pregnancy tests and monitoring drug levels in the body.