Lymphocytic leukemia is a cancer of the bone marrow. It can be a rapidly developing and more aggressive form of disease called acute lymphocytic leukemia (ALL) or a slowly developing and less aggressive form called chronic lymphocytic leukemia (CLL). In both of these forms, the bone marrow produces an excess number of abnormal lymphocytes (a type of white blood cell) that under normal conditions are responsible for fighting infection by producing antibodies (proteins that fight infection). These leukemic cells (malignant white blood cells), known as lymphoblasts in acute leukemia, adversely affect immunological response (ability to fight infections) and can accumulate in different organs such as lymph nodes, spleen, liver, brain, and spinal cord, impairing their functions. ALL and CLL affect mostly older white men but also individuals who were exposed to radiation or previous chemotherapy, have certain genetic disorders (such as Down syndrome or chromosome 22 Philadelphia), or are of Eastern European Jewish ancestry. The January 5, 2011, issue of JAMA includes an article describing factors that can affect the course of CLL.