Lymphoma is cancer that 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 you have lymphoma, lymphocytes change and grow out of control. The lymph system is a series of lymph nodes and vessels that move lymph fluid through the body. Lymph fluids contain infection-fighting white blood cells. Lymph nodes act as filters, capturing and destroying bacteria and viruses to prevent infection from spreading. While the lymph system typically protects your body, lymph cells called lymphocytes can become cancerous. The names for cancers that occur in the lymph system are lymphomas. Doctors classify more than 70 cancer types as lymphomas. Cancer is the result of uncontrolled cell growth. The average lifespan of a cell is brief, and then the cell dies. In people with lymphoma, however, the cell thrives and spreads instead of dying. It’s unclear what causes lymphoma, but a number of risk factors are connected with these cancers.