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. Lymphoma is a cancer of the lymphatic system. It develops in lymphocytes, which are a type of white blood cell. These cells help fight disease in the body and play an essential role in the body’s immune defenses. As this type of cancer is present in the lymph system, it can quickly metastasize, or spread, to different tissues and organs throughout the body. Lymphoma most often spreads to the liver, bone marrow, or lungs. People of any age can develop lymphoma, but it is among the most common causes of cancer in children and young adults aged 15–24 years. It is often treatable. There are two main types of lymphoma: Hodgkin and non-Hodgkin lymphoma.