Lymphocytic Leukocytosis

ByDavid C. Dale, MD, University of Washington
Reviewed/Revised Apr 2023

    Lymphocytic leukocytosis is an abnormally high number of lymphocytes (a type of white blood cell) in the blood.

    Lymphocytes are a type of white blood cell that play several roles in the immune system, including protection against bacteria, viruses, fungi, and parasites.

    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, may also increase the number. Certain types of cancer, such as lymphomas and acute or chronic lymphocytic leukemia, 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 and 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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