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.

Myeloid Leukemia Versus Lymphocytic Leukemia

The second factor to take into account is the type of bone marrow cells that are involved.

Myeloid leukemia: Myeloid leukemia mainly develops from white blood cells such as granulocytes or monocytes, but can also form from the cells that mature into platelets or red blood cells.

Lymphocytic leukemia: Lymphocytic leukemias develop from lymphocytes (a type of white blood cell) in the bone marrow.

Most cases of leukemia can be sorted into 1 of the 4 main types shown in the table below.

Acute lymphocytic leukemia (ALL)	Acute myeloid leukemia (AML)
Chronic lymphocytic leukemia (CLL)	Chronic myeloid leukemia (CML)

Although ALL is the most common of the 4 major types of leukemia among children, it is the least common type among adults.

The rest of this document contains information on ALL of adults only. Chronic leukemias of adults and acute myelocytic leukemia (AML) of adults are discussed in other American Cancer Society documents. For information about ALL in children, please see the separate document, Childhood Leukemias.