It is a branch of immunology that deals with the interaction of immune system with the tumor cells. Tumor immunology is a growing field of research which aims to discover new and innovative tumor or cancer immunotherapies that can be used to retard or treat the progression of tumor. The immune response, which includes the recognition of tumor-specific antigens, is important as it will help in the development of targeted therapy and tumor marker-based diagnostic tests. The immune system can respond to tumor cells in two ways: by reacting against tumor-specific antigens (molecules that are unique to cancer cells) or by reacting against tumor-associated antigens (molecules that are expressed differently by cancer cells and normal cells). Tumors that are caused by viruses display viral antigens which serve as tumor antigens. The expression of molecules of cell surface give rise to tumor antigens on the surface of the tumor cell. Monoclonal antibodies which react specifically with tumor cells are used to characterize putative human tumor antigens. Some tumor antigens consist of those molecules which are unique to the tumor cell but not similar to the normal cell. Some tumor antigens are sometimes qualitatively same as those found on normal cells but are over expressed on the tumor cell.