Cancer immunotherapy (sometimes called immuno-oncology) is the artificial stimulation of the immune system to treat cancer, improving on the system's natural ability to fight cancer. It is an application of the fundamental research of cancer immunology and a growing subspecialty of oncology. It exploits the fact that cancer cells often have tumor antigens, molecules on their surface that can be detected by the antibody proteins of the immune system, binding to them. The tumor antigens are often proteins or other macromolecules (e.g. carbohydrates). Normal antibodies bind to external pathogens, but the modified immunotherapy antibodies bind to the tumor antigens marking and identifying the cancer cells for the immune system to inhibit or kill.