Lymphoma cancer cells. Coloured scanning electron micrograph (SEM) of lymphoma cells. A lymphoma is a cell of the immune system that has become cancerous. The cell becomes immortal and can grow indefinitely. A number of these cells will form a tumour. Lymphomas most commonly occur in the lymph nodes and spleen, which are rich in tissue containing lymphocytes, and can spread to the liver and bone marrow. Lymphoma cancers are classified into either Hodgkin's lymphoma (presence of Reed-Sternberg cells) or non- Hodgkin's lymphoma. Treatment is with chemotherapy and radiation therapy and is often successful. Magnification: x2000 when printed at 10 centimetres high. Specimen courtesy of Professor Greg Towers, University College London, UK