Cancerous cells often produce tumor markers as well as its own cancer cells that act as antigens that can be recognized by antigen presenting cells and be presented to natural killer (NK) cell progenitors that are found in the spleen, lymph nodes, thymus, and other lymphoid organs. With malignancy, the production of NK cells will be increased. NK cells works by killing cells that do not have an inhibitory signal (i.e. cancer cells) and preserving cells that do have an inhibitory signal (i.e. normal body cells).