Health A-Z

Medical Content Created by the Faculty of the Harvard Medical School


What Is It?

Chronic lymphocytic leukemia (CLL) is a cancer that occurs when the bone marrow makes too many lymphocytes, a type of white blood cell. CLL usually grows slowly compared to other leukemias, and it may not cause symptoms for some time.

CLL is one of four main types of leukemia. It is the second most common type of leukemia in adults; most people with CLL are middle-aged or older. The disease is very rare in children.

Normally, a person's immature blood stem cells develop into myeloid and lymphoid stem cells. The myeloid cells become mature blood cells: white blood cells, red blood cells, and platelets. Lymphoid stem cells develop into three types of infection-fighting lymphocytes:

  • B lymphocytes, which make antibodies to help protect the body from germs

  • T lymphocytes, which can destroy virus-infected cells, foreign cells, and cancer cells

  • Natural killer cells, which also can kill cancer cells and viruses.

In CLL, too many blood stem cells turn into abnormal lymphocytes. These cells do not function properly. Even in high numbers, they cannot fight infection as well as normal cells. As they pile up in the blood and bone marrow, they crowd out healthy white blood cells, red blood cells, and platelets. This can result in infection, anemia, and easy bleeding. CLL most often develops from B lymphocytes.

CLL begins in the bone marrow but then typically invades the blood. In time, it can spread to other parts of the body that normally contain lymphocytes, such as the lymph nodes (in the neck, abdomen, and groin, under the arms, and around the collarbone), as well as the liver and the spleen. These tissues may then become enlarged.

Certain factors may increase a person's chance of developing CLL. These include

  • having relatives with CLL or cancer of the lymph system

  • being exposed to certain chemicals

  • being middle-aged or older

  • being male and white

  • being of North American or European descent.

Keep in mind, however, that having one or more risk factors for CLL does not mean you will develop the disease.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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