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The bone marrow is the spongy inner part of bones in which all of the different types of blood cells are made. Early (primitive) blood cells are called stem cells. As these stem cells mature, they develop into either red blood cells white blood cells (leukocytes), or platelets. There are three main types of white blood cells: granulocytes, monocytes, and lymphocytes.

• Red blood cells are responsible for carrying oxygen from the lungs to all the tissues in the body and carbon dioxide from the tissues back to the lungs
• White blood cells are important in fighting infection
• Platelets play a critical role in blood clotting

Leukemia is a cancer of the white blood cells that arises from the bone marrow and circulates in the peripheral blood. It is characterized by uncontrolled growth of white blood cells. Leukemia is a disease of the blood cells and does not usually form a solid mass or tumor. Although leukemia starts in the bone marrow, it can spread to the blood, lymph nodes, spleen, liver, central nervous system (CNS) and other organs.

There are four major types of leukemias:

• Acute myelogenous leukemia (AML)
• Chronic myelogenous leukemia (CML)
• Acute lymphocytic leukemia (ALL)
• Chronic lymphocytic leukemia (CLL)

The terms "myelogenous" and "lymphocytic" denote the different types of cell involved. The terms "acute" or "chronic" refer to the rate of progression of the disease.

Acute leukemia is a rapidly progressing disease that results in the accumulation of immature, non-functional cells in the marrow and blood. As a result, the bone marrow often can no longer produce enough normal red blood cells, white blood cells, and platelets. Chronic leukemia progresses more slowly and permits greater numbers of more mature functional cells to be made.

According to the American Cancer Society, approximately 15,300 new cases of chronic lymphocytic leukemia (CLL) will be diagnosed in 2007 in the United States. Chronic lymphocytic leukemia affects males more than females with men accounting for about 60% of all CLL cases. The incidence of CLL is much higher in Western countries than in Asian countries such as China and Japan.

Chronic lymphocytic leukemia represents approximately 25% of all leukemias and occurs most frequently in the elderly population. The median age at the time of diagnosis is 70. Chronic lymphocytic leukemia usually does not develop in people under age 40 and is also extremely rare in children The incidence of CLL in the United States is higher in Caucasians than in African Americans. Recent research indicates that the 5-year survival rate of patients with CLL is 73%. The American Cancer Society estimates that about 4,500 people in the U.S. will die of CLL in 2007.

Chronic lymphocytic leukemia affects a type of lymphocyte called the B lymphocytes. Lymphocytes are specialized cells of the immune system that play a major role in the body's ability to fight-off infections. There are 2 major categories of lymphocytes:

• B lymphocytes - These lymphocytes are antibody producing cells that are responsible for a type of immunity known as "humoral immunity"