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Lymphocytosis        REF:  From Wikipedia, the free encyclopedia                                    

Lymphocytosis is an increase in the number or proportion of lymphocytes in the blood, usually detected when a complete blood count is routinely obtained. Lymphocytes normally represent 20 to 40% of circulating white blood cells. The absolute lymphocyte count (ALC) can be directly measured by flow cytometry, or calculated by multiplying the total white blood cell (WBC) count by the percentage of lymphocytes found in the differential count[1]. In absolute lymphocytosis, the total lymphocyte count is elevated above the population reference range (normal).

In adults, absolute lymphocytosis is present when the absolute lymphocyte count is greater than 4000 per microliter, in older children greater than 7000 per microliter and in infants greater than 9000 per microliter[2]. Relative lymphocytosis occurs when there is a higher proportion (greater than 40%) of lymphocytes among the white blood cells, however, absolute lymphocyte count (ALC) is normal (less than 4000 per microliter. Relative lymphocytosis is normal in children under age 2.

Lymphocytosis is a feature of infection, particularly in children. In the elderly, lymphoproliferative disorders, including chronic lymphocytic leukaemia and lymphomas, often present with lymphadenopathy and a lymphocytosis.

Causes of absolute lymphocytosis include:

Causes of relative lymphocytosis include:

age less than 2 years; acute viral infections; connective tissue diseases, thyrotoxicosis, Addison's disease, and splenomegaly with splenic sequestration of granulocytes.


  1. 1.^ For example, if the total WBC count is 30,000, and the %lymphocytes is 30, the absolute lymphocyte count is 9,000 per microliter.
  2. 2.^ Miale, John B. Laboratory Medicine: hematology. 5th. St. Louis: C.V. Mosby, 1977.
  3. 3.^ a b c d e f Table 12-6 in: Mitchell, Richard Sheppard; Kumar, Vinay; Abbas, Abul K.; Fausto, Nelson. Robbins Basic Pathology. Philadelphia: Saunders. ISBN 1-4160-2973-7. 8th edition.