Light micrograph section through human tissue showing lymphoma, a blood cancer that develops from blood cells. This cancer begins in infection-fighting cells of the immune system, called lymphocytes. These cells are in the lymph nodes, spleen, thymus, bone marrow and other parts of the body. When people have lymphoma, lymphocytes change and grow out of control. There are two types of lymphoma: Non-Hodgkin (most people with lymphoma have this type) and Hodgkin.