Leukemia is a cancer of white blood cells. In acute leukemia, the cancerous cells are immature forms called blasts that cannot properly fight infection; patients become ill in rapid fashion.
The cells that make up blood are produced in the bone marrow and the lymph system. The bone marrow is the spongy tissue found in the large bones of the body. The lymph system includes the spleen (an organ in the upper abdomen), the thymus (a small organ beneath the breast-bone), and the tonsils (an organ in the throat). In addition, the lymph vessels (tiny tubes that branch like blood vessels into all parts of the body) and lymph nodes (pea-shaped organs that are found along the network of lymph vessels) are also part of the lymph system. The lymph is a milky fluid that contains cells. Clusters of lymph nodes are found in the neck, underarm, pelvis, abdomen, and chest.
The main types of cells found in the blood are the red blood cells (RBCs), which carry oxygen and other materials to all tissues of the body; white blood cells (WBCs), which fight infection; and the platelets, which play a part in the clotting of the blood. The white blood cells can be further subdivided into three main types: granulocytes, monocytes, and lymphocytes.
The granulocytes, as their name suggests, have particles (granules) inside them. These granules contain special proteins (enzymes) and several other substances that can break down chemicals and destroy microorganisms such as bacteria. Monocytes are the second type of white blood cell. They are also important in defending the body against pathogens. The lymphocytes form the third type of white blood cell. The two types of lymphocytes are B-cells, which make antibodies, and T-cells, which make other infection-fighting substances. Lymphocytic leukemia can arise in either B or T cells.
B-cell leukemia occurs more frequently than T-cell leukemia. It is the most common form of leukemia in children, but also occurs in adults. At diagnosis, leukemic cells can be found throughout the body, in the bloodstream, the lymph nodes, spleen, liver, occasionally in the central nervous system, and in T-cell ALL, the thymus gland.
Cancerous lymphoblasts take over the bone marrow, reducing both the number and the effectiveness of all types of blood cells. The cancerous cells reduce the ability of healthy white cells to fight infection. Fewer red cells are produced, causing anemia, and fewer platelets increases the risk of bleeding and bruising. The presence of the cancerous white cells in the central nervous system can produce headaches, confusion and seizures.
The type of treatment a person receives for ALL depends on the presence of risk factors for relapse. Children are at standard risk if they are between ages 1 and 9, have a total white cell count of less than 50, 000 per microliter of blood, and have B-precursor cell leukemia. Children are at high risk if they are younger than 1 or older than 9, if their white blood cell count exceeds 50, 000 per microliter, or if they have T-cell leukemia. Compared to children, adults are all at higher risk of relapse at the time of diagnosis, but younger adults (less than 25 years old) have a better prognosis.
B-cell ALL constitutes about 80% of all cases. The cancerous cells are either early pre-B cells, the most immature, pre-B cells, also somewhat immature, or B-cells. These B-lineage cells contain a variety of proteins called antigens. The presence of one of these antigens, called CALLA for common ALL antigen, carries a somewhat more favorable prognosis.
T-cell ALL has a less favorable prognosis than B-cell ALL. The presence of an antigen called CD2 indicates a more favorable prognosis.
ALL is also classified by karyotype, which is the number and composition of a cell's chromosomes. Normal human cells contain 46 chromosomes. One chromosomal abnormality often seen in ALL is a translocation, in which a piece of one chromosome becomes attached to a different chromosome. Different translocations carry different prognoses. One translocation, labeled t(9;22) is also called the Philadelphia chromosome and is found in 5% of childhood ALL and 20% of adult ALL cases. The Philadelphia chromosome carries a somewhat less favorable prognosis.
The number of chromosomes found in the leukemic cells, particularly in children, also impacts prognosis. The occurrence of more than 50 chromosomes in leukemic cells has a very favorable prognosis. Even the presence of one extra chromosome can be favorable.
Children whose leukemic cells have fewer than 45 chromosomes are at highest risk of treatment failure.
Marianne Vahey M.D., The Gale Group Inc., Gale, Detroit,