Cells of the immune system, like those of other body systems, can proliferate uncontrollably; the result is cancer. Leukemias are caused by the proliferation of white blood cells, or leukocytes. The uncontrolled growth of antibody-producing (plasma) cells can lead to multiple myeloma. Cancers of the lymphoid organs, known as lymphomas, include Hodgkin's disease. These disorders can be treated-some of them very successfully-by drugs and/or irradiation.