Introduction:

The word leukemia literally means "white blood" and is used to describe a variety of cancers that begin in the blood-forming cells of the bone marrow.

White blood cells (leukocytes) evolve from immature cells referred to as blasts. Malignancy in these blasts is the source of leukemias, which generally progresses as follows:

• Normally, blasts constitute 5% or less of healthy bone marrow. In leukemia, however, these blasts remain abnormally immature and multiply continuously, eventually constituting between 30 - 100% of the bone marrow.
• Eventually these malignant blast cells fill up the bone marrow and prevent production of healthy red cells, platelets, and mature white cells (leukocytes).

They spill out of the marrow into the bloodstream and lymph system and can travel to the brain and spinal cord (the central nervous system). As the number of normal cells decline, dangerous symptoms develop, which, if untreated, become lethal.

Leukemias are divided into two major types:

• Acute (which progresses quickly with many immature white cells)
• Chronic (which progresses more slowly and has more mature white cells)

Some blasts are called lymphoblasts (which become mature cells called lymphocytes) and others are called myeloblasts (which mature to myeloid cells). Acute leukemias are in turn subdivided into two classifications according to whether the malignant blasts are lymphocytes or myeloid:

• Acute lymphocytic leukemia (ALL), which is the subject of this report
• Acute myeloid leukemia (AML), which is not covered in this report

Acute Lymphocytic Leukemia

Acute lymphocytic leukemia (ALL) is also known as acute lymphoid leukemia or acute lymphoblastic leukemia. The majority of childhood leukemias are of the ALL type. Malignancies in this disease can arise either in T-cell or B-cell lymphocytes.

• T cell ALL is diagnosed in 15% of children and adults with ALL.
• About 85% of ALL cases are of the B-cell lymphocyte lineage (often referred to as "early" or "pre" B-cell lineage).

Blood Cell Lines and the Lymph System

Blood Cell Lines

In adults, blood cells are produced by the bone marrow, the spongy material filling the body's bones. The bone marrow produces two blood cell groups, myeloid and lymphoid.

Myeloid Cell Line. The myeloid cell line includes the following:

• Immature cells called erythrocytes that later develop into red blood cells
• Blood clotting cells (platelets)
• Some white blood cells, including macrophages (which act as scavengers for foreign particles), eosinophils (which trigger allergies and also defend against parasites), and neutrophils (the main defenders against bacterial infections)

Lymphoid Cell Line. The lymphoid cell line includes the lymphocytes, which are the body's primary infection fighters. Among other vital functions, certain lymphocytes are responsible for producing antibodies, factors that can target and attack specific foreign substances (antigens).

Lymphocytes develop in the thymus gland or bone marrow and are therefore categorized as either B cells (bone marrow-derived cells) or T cells (thymus gland-derived cells).

Lymphocytes and the Lymph System

Understanding how acute lymphocytic leukemia (ALL) arises requires knowledge of lymphocytic development and function:

• B cells develop and mature in their final form (known as differentiation) in the bone marrow.
• T cells also start out in the bone marrow but differentiate and mature in the thymus gland, located beneath the breastbone. This small gland is active mostly in the fetal stage through the first 10 years of life, after which it atrophies (shrinks).
• B-cell and T-cell lymphocytes leave these organs through the bloodstream, which eventually branches out into the tiny blood vessels called capillaries.
• Once they leave the capillaries, some lymphocytes migrate into the surrounding tissues. Some of these lymphocytes (along with fluid, proteins, and other substances) then enter the lymphatic vessels.
• Lymphatic vessels begin as tiny, blind-ended tubes and lead to larger lymphatic ducts and branches. They drain into two ducts in the neck, where the fluid re-enters the bloodstream.
• Along the way, the fluid passes through lymph nodes, which are oval structures composed of lymph vessels, connective tissue, and white blood cells. Here, the lymphocytes are either filtered out or are added to the contents of the node.