| Leukemia is a type of cancer that starts in the soft, inner part of the bones (bone marrow) and often moves quickly into the blood. It can then spread to other parts of the body such as the lymph nodes, the spleen, liver, central nervous system and other organs. In contrast, other types of cancer can start in these organs and then spread to the bone marrow (or elsewhere). Those cancers are not leukemia. Both children and adults can get leukemia. The term "acute" means that the cancer develops quickly, and if not treated, could be fatal in a few months. "Lymphocytic" (limf-o-sit-ik) means that the cancer starts from cells called lymphocytes (limf-o-sites). Acute lymphocytic leukemia (ALL) is a type of cancer that starts from these white blood cells in the bone marrow. ALL is sometimes called acute lymphoblastic leukemia. Leukemia is a complex disease with many different types and sub-types. The kind of treatment given and the outlook for the person with leukemia vary greatly according to the exact type of leukemia and other factors. Normal Bone Marrow, Blood, and Lymphoid Tissue In order to understand the different types of leukemia, it helps to have some basic knowledge of the blood and lymph systems. Bone Marrow Bone marrow is the soft, spongy, inner part of bones. All of the different types of blood cells are made in the bone marrow. Bone marrow is made up of blood stem cells, blood-forming cells, fat cells, and tissues that aid the growth of blood cells. The blood-forming cells come from blood stem cells. They only make new blood-forming cells and not other kinds of cells. (They are different from embryonic stem cells which are formed from a developing fetus and can grow to become other kinds of cells in the body.) The blood-forming cells can develop into 1 of the 3 main types of blood cell: red blood cells, white blood cells, or platelets. Red Blood Cells Red blood cells carry oxygen from the lungs to all other tissues of the body. They also carry away carbon dioxide, a waste product of cell activity. A shortage of red blood cells causes weakness, shortness of breath, and tiredness. Platelets Platelets are actually pieces that break off from certain bone marrow cells.. Platelets help stop bleeding by plugging up holes in blood vessels caused by cuts or bruises. A shortage of platelets can cause a person to bleed or bruise easily. White Blood Cells White blood cells help defend the body against germs such as viruses and bacteria. There are quite a few types of white blood cells. Each has a special role to play in protecting the body against infection. The 3 main types of white blood cells are granulocytes, monocytes, and lymphocytes. The immune system is made up mainly of lymphoid tissue (also known as lymphatic tissue). The main cell type that forms lymphoid tissue is the lymphocyte, a kind of white blood cell. The two main types of lymphocytes are called B-cells and T-cells. Normal T-cells and B-cells do different jobs within the immune system. Any of the blood-forming cells can turn into a leukemic cell. Once that happens, the cell can reproduce to form many new cancer cells. These cells can overwhelm the bone marrow, spill out into the bloodstream, and spread to other organs. Acute lymphocytic leukemia starts from early forms of the lymphocytes. Types of Leukemia Not all leukemias are the same. Leukemias are divided into 4 main types. Knowing the exact type of leukemia can help doctors better predict each patient's outlook (prognosis) and select the best treatment. The major types of leukemia are based on these factors: - acute or chronic
- lymphocytic or myeloid
Acute Leukemia Versus Chronic Leukemia The first factor to take into account to classify a patient's leukemia is whether most of the abnormal cells look like normal mature white blood cells or whether they look more like stem cells (have not matured.) Acute leukemia: In acute leukemia, the bone marrow cells don’t mature properly. These immature cells build up and crowd out normal cells. Without treatment, most patients with acute leukemia would live only a few months. Some types of acute leukemia respond well to treatment and many patients are cured. People with other types often do not do as well. Chronic leukemia: In chronic leukemia the cells look mature, but they are not really normal and they can’t fight infection they way they should. They also live too long, so that they build up and crowd out normal bone marrow cells. Chronic leukemias tend to progress over a longer period of time, and most patients can live for many years. However, chronic leukemias are generally harder to cure than acute leukemias. | 
