Antibodies manufactured in the lab and designed to seek out as targets specific substances, called antigens, recognized by the immune system. Monoclonal antibodies which have been attached to chemotherapy drugs or radioactive substances are being studied for their potential to seek out antigens unique to cancer cells and deliver these treatments directly to the cancer, thus killing the cancer cells and not harming healthy tissue. Monoclonal antibodies are often used in immunocytochemistry to help detect and classify cancer cells. Other studies are being done to see if radioactive atoms attached to monoclonal antibodies can be used in imaging tests to detect and locate small groups of cancer cells.