Lymphoma: is cancer that starts in infection-fighting cells of the immune system, called lymphocytes. These cells are in the lymph nodes, thymus, spleen, bone marrow, and other parts of the body. When you have lymphoma, lymphocytes change and grow out of control. Lymphoma is very treatable, and the outlook can vary or be built upon on the type of lymphoma and its stage. Your doctor can help you find the right treatment for your type and stage of the illness.