Leukemia

What Is Acute Lymphocytic Leukemia (ALL)?Acute lymphocytic leukemia (ALL), also called acute lymphoblastic leukemia, is a cancer that starts from white blood cells called lymphocytes in the bone marrow (the soft inner part of the bones, where new blood cells are made).

In most cases, the leukemia invades the blood fairly quickly. It can then spread to other parts of the body, including the lymph nodes, liver, spleen, central nervous system (brain and spinal cord), and testicles (in males). Other types of cancer that start in these organs and then spread to the bone marrow are not leukemia.

The term "acute" means that the leukemia can progress quickly, and if not treated, would probably be fatal in a few months. "Lymphocytic" or "lymphoblastic" means it develops from cells called lymphocytes or lymphoblasts. This is different from acute myeloid leukemia (AML), which develops in other blood cell types found in the bone marrow. For more information on AML, see our document, Leukemia--Acute Myeloid.

Other types of cancer that start in lymphocytes are known as lymphomas (non-Hodgkin lymphoma or Hodgkin disease). The main difference between these types of cancers is that ALL starts in the bone marrow and may spread to other places, while lymphomas start in lymph nodes or other organs and then may spread to the bone marrow. Sometimes cancerous lymphocytes are found in both the bone marrow and lymph nodes when the cancer is first diagnosed, which can make it hard to tell if the cancer is a leukemia or a lymphoma. If more than 25% of the bone marrow is replaced by cancerous lymphocytes, the disease is usually considered to be a leukemia. The size of lymph nodes is also important. The bigger they are, the more likely the disease is a lymphoma. For more information on lymphomas, see our documents, Non-Hodgkin Lymphoma and Hodgkin Disease.

Normal bone marrow, blood, and lymphoid tissue

In order to understand the different types of leukemia, it helps to know about the blood and lymph systems.

Bone marrow

Bone marrow is the soft inner part of some bones, such as the skull, shoulder blades, ribs, pelvis, and backbones. The bone marrow is made up of a small number of blood stem cells, more mature blood-forming cells, fat cells, and supporting tissues that help cells grow.

Blood stem cells go through a series of changes to make new blood cells. During this process, the cells develop into either lymphocytes (a kind of white blood cell) or other blood-forming cells. The blood-forming cells can develop into 1 of the 3 main types of blood cell components:

• red blood cells
• white blood cells (other than lymphocytes)
• platelets

Red blood cells

Red blood cells carry oxygen from the lungs to all other tissues in the body, and take carbon dioxide back to the lungs to be removed. Anemia (having too few red blood cells in the body) typically causes a person to feel tired, weak, and short of breath because the body tissues are not getting enough oxygen.

Platelets

Platelets are actually cell fragments made by a type of bone marrow cell called the megakaryocyte. Platelets are important in plugging up holes in blood vessels caused by cuts or bruises. A shortage of platelets is called thrombocytopenia. A person with thrombocytopenia may bleed and bruise easily.

White blood cells

White blood cells help the body fight infections. Lymphocytes are one type of white blood cell. The other types of white blood cells are granulocytes (neutrophils, basophils, and eosinophils) and monocytes.

Lymphocytes: These are the main cells that make up lymphoid tissue, a major part of the immune system. Lymphoid tissue is found in lymph nodes, the thymus gland, the spleen, the tonsils and adenoids, and is scattered throughout the digestive and respiratory systems and the bone marrow.

Lymphocytes develop from cells called lymphoblasts to become mature, infection-fighting cells. The 2 main types of lymphocytes are B lymphocytes (B cells) and T lymphocytes (T cells).

• B lymphocytes: B lymphocytes protect the body from invading germs by developing (maturing) into plasma cells, which make proteins called antibodies. The antibodies attach to the germs (bacteria, viruses, and fungi), which helps other white blood cells called granulocytes to recognize and destroy them.

• T lymphocytes: T lymphocytes can recognize cells infected by viruses and directly destroy these cells.

Granulocytes: These are white blood cells that have granules in them, which are spots that can be seen under the microscope. These granules contain enzymes and other substances that can destroy germs, such as bacteria. The 3 types of granulocytes -- neutrophils, basophils, andeosinophils -- are distinguished by the size and color of their granules. Granulocytes develop from blood-forming cells called myeloblasts to become mature, infection-fighting cells.

Monocytes: These white blood cells, which are related to granulocytes, also help protect the body against bacteria. They start in the bone marrow as blood-forming monoblastsand develop into mature monocytes. After circulating in the bloodstream for about a day, monocytes enter body tissues to become macrophages, which can destroy some germs by surrounding and digesting them. Macrophages also help lymphocytes recognize germs and start making antibodies to fight them.

Any of the blood-forming or lymphoid cells from the bone marrow can turn into a leukemia cell. Once this change takes place, the leukemia cells fail to go through their normal process of maturing. Leukemia cells may reproduce quickly, but in most cases they don't die when they should. They survive and build up in the bone marrow. Over time, these cells spill into the bloodstream and spread to other organs, where they can keep other cells in the body from functioning normally.

Types of leukemia

Not all leukemias are the same. Leukemias are divided into 4 main types. Knowing the specific type of leukemia helps doctors better predict each patient's prognosis (outlook) and select the best treatment.

Acute leukemia versus chronic leukemia

The first factor in classifying a patient's leukemia is whether most of the abnormal cells are mature (look like normal white blood cells) or immature (look more like stem cells).

Acute leukemia: In acute leukemia, the bone marrow cells cannot mature properly. Immature leukemia cells continue to reproduce and build up. 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 can be cured. Other types of acute leukemia have a less favorable outlook.

Chronic leukemia: In chronic leukemia, the cells can mature partly but not completely. These cells may look fairly normal, but they are not. They generally do not fight infection as well as do normal white blood cells. And they survive longer, build up, and crowd out normal cells. Chronic leukemias tend to progress over a longer period of time, and most patients can live for many years. But chronic leukemias are generally harder to cure than acute leukemias.

Myeloid leukemia versus lymphocytic leukemia

The second factor in classifying leukemia is the type of bone marrow cells that are affected.

Myeloid leukemia: Leukemias that start in early forms of myeloid cells -- white blood cells (other than lymphocytes), red blood cells, or platelet-making cells (megakaryocytes) -- aremyeloid leukemias (also known as myelocytic, myelogenous, or non-lymphocytic leukemias).

Lymphocytic leukemia: If the cancer starts in early forms of lymphocytes, it is called lymphocytic leukemia (also known aslymphoid or lymphoblastic leukemia). Lymphomas are also cancers that start in lymphocytes. Lymphocytic leukemias develop from cells in the bone marrow, but lymphomas develop from cells in lymph nodes or other organs.

Leukemias can be divided into 4 main types based on whether they are acute or chronic, and whether they are myeloid or lymphocytic. The 4 main types are:

• acute myeloid (or myelogenous) leukemia (AML)
• chronic myeloid (or myelogenous) leukemia (CML)
• acute lymphocytic (or lymphoblastic) leukemia (ALL)
• chronic lymphocytic leukemia (CLL)

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