Dividing lymphoma cancer cells. Coloured scanning electron micrograph (SEM) of a lymphoma cell replicating. 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. The parent cell passes on the defects that caused it to become malignant to the daughter cell. Lymphomas most commonly occur in the lymph nodes and spleen, which are rich in tissue containing lymphocytes. Lymphoma cancers are classified into either Hodgkin's lymphoma (presence of Reed-Sternberg cells) or non-Hodgkin's lymphoma. Treatment is with chemotherapy or radiation therapy.