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Lymphocytic Leukocytosis


David C. Dale

, MD, University of Washington

Reviewed/Revised Apr 2023

There are three types of lymphocytes

  • B cells (B lymphocytes)

  • T cells (T lymphocytes)

  • Natural killer cells

All three types can be increased in response to infections or cancer. However, in some cases only a specific type of lymphocyte is increased.

The most common cause of an increased number of lymphocytes is

Some bacterial infections, such as tuberculosis Tuberculosis (TB) Tuberculosis is a chronic contagious infection caused by the airborne bacteria Mycobacterium tuberculosis. It usually affects the lungs, but almost any organ can be involved. Tuberculosis... read more Tuberculosis (TB) , may also increase the number. Certain types of cancer, such as lymphomas Overview of Lymphoma Lymphomas are cancers of lymphocytes, which reside in the lymphatic system and in blood-forming organs. Lymphomas are cancers of a specific type of white blood cells known as lymphocytes. These... read more Overview of Lymphoma and acute or chronic lymphocytic leukemia Overview of Leukemia Leukemias are cancers of white blood cells or of cells that develop into white blood cells. White blood cells develop from stem cells in the bone marrow. Sometimes the development goes awry... read more , may produce an increase in the number of lymphocytes, in part by releasing immature lymphocytes (lymphoblasts) or the lymphoma cells into the bloodstream. Graves disease Hyperthyroidism Hyperthyroidism is overactivity of the thyroid gland that leads to high levels of thyroid hormones and speeding up of vital body functions. Graves disease is the most common cause of hyperthyroidism... read more Hyperthyroidism and Crohn disease Crohn Disease Crohn disease is an inflammatory bowel disease where chronic inflammation typically involves the lower part of the small intestine, the large intestine, or both and may affect any part of the... read more Crohn Disease may also result in an increase in the number of lymphocytes in the bloodstream.

The increased number of lymphocytes usually does not cause symptoms. However, in people with lymphoma and certain leukemias, the increase in lymphocytes may cause fever, night sweats, and weight loss. Also, symptoms may result from the infection or other disease that has caused the number of lymphocytes to increase, rather than from the increase in lymphocytes per se.

When an infection is suspected, doctors may do blood tests. When doctors discover an increased number of lymphocytes, a blood sample is examined under a microscope to determine if the lymphocytes in the blood appear activated (as occurs in response to viral infections) or if they appear immature or abnormal (as occurs in certain leukemias or lymphomas). Blood tests can also identify the specific type of lymphocyte (T cells, B cells, natural killer cells) that is increased to help determine the underlying problem.

Treatment for lymphocytic leukocytosis depends on the cause.

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