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Approved by the Cancer.Net Editorial Board, 10/2017

About leukemia

Leukemia is a cancer of the blood. Leukemia begins when healthy blood cells change and grow out of control. Chronic lymphocytic leukemia (CLL) is a cancer of the lymphocytes. Lymphocytes are a type of white blood cell involved in the body’s immune system. In some people with CLL, the disease grows and progresses slowly. This means it may take years for symptoms to appear or for treatment to be needed. In fact, some patients may never need treatment for their CLL. In other patients, the disease grows more quickly and needs treatment sooner.

About lymphocytes

Lymphocytes circulate in the bloodstream and are made in 4 places in the body:

  • Lymph nodes, which are the tiny, bean-shaped organs that fight infection
  • Spleen, which also filters the blood
  • Thymus, an organ under the breast bone
  • Bone marrow, the spongy, red tissue in the inner part of the large and flat bones

There are 3 different types of lymphocytes:

  • T cells, which fight infection by triggering other cells in the immune system and by destroying infected cells
  • B cells, which make antibodies
  • Natural killer (NK) cells, which fight microbes and cancer cells

About CLL

In people with CLL, the abnormal cells crowd other types of cells in the bone marrow. This crowding prevents the production of the healthy blood cells, including:

  • Red blood cells that carry oxygen
  • Other types of white blood cells, such as neutrophils or granulocytes that fight infection
  • Platelets, which are needed for blood to clot

This means that people with CLL may have anemia from low levels of red blood cells and more infections because they do not have enough white blood cells. They may also bruise or bleed more easily because of a low level of platelets.

Most often, CLL is diagnosed when too many abnormal lymphocytes are found in the blood, also known as lymphocytosis. However, the same disease can occur when the abnormal lymphocytes are mostly in the lymph nodes but not in the blood. This is called small lymphocytic lymphoma, but it behaves very similarly to CLL.

Types of CLL

There are 2 general types of CLL based on whether the disease affects B cells or T cells. It is important for doctors to find out whether the disease is caused by the overgrowth of T cells or B cells.

However, even between these 2 types of CLL, there are several subtypes that differ at the genetic level. This means that the disease may act differently based on the genetic subtype. For example, one person with the B-cell type may have the disease act differently than another person with the B-cell type.

Learn more about other, rare types of chronic T-cell leukemia and types of B-cell leukemia.