lymphoma is a disease in which malignant (cancer) cells form in the lymph system. The lymph system is part of the immune system and is made up of the following: Lymph: Colorless, watery fluid that travels through the lymph system and carries white blood cells called lymphocytes. Lymphocytes protect the body against infections and the growth of tumors. Lymph vessels: A network of thin tubes that collect lymph from different parts of the body and return it to the bloodstream. Lymph nodes: Small, bean-shaped structures that filter substances in lymph and help fight infection and disease. Lymph nodes are located along the network of lymph vessels found throughout the body. Clusters of lymph nodes are found in the underarm, pelvis, neck, abdomen, and groin. Spleen: An organ that produces lymphocytes, filters the blood, stores blood cells, and destroys old blood cells. It is located on the left side of the abdomen near the stomach. Thymus: An organ in which lymphocytes grow and multiply. The thymus is in the chest behind the breastbone. Tonsils: Two small masses of lymph tissue at the back of the throat. The tonsils produce lymphocytes. Bone marrow: The soft, spongy tissue in the center of large bones. Bone marrow produces white blood cells, red blood cells, and platelets.