|Classification and external resources|
Lymphocytosis, peripheral blood smear (40x)
Lymphocytosis is an increase in the number or proportion of lymphocytes in the blood. In absolute lymphocytosis, the total lymphocyte count is elevated. 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. Lymphocytes normally represent 20 to 40% of circulating white blood cells. Relative lymphocytosis occurs when there is a higher proportion (greater than 40%) of lymphocytes among the white blood cells, while the absolute lymphocyte count (ALC) is normal (less than 4000 per microliter). Relative lymphocytosis is normal in children under age 2.
Lymphocytosis is usually detected when a complete blood count is routinely obtained. 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.
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:
- acute viral infections, such as infectious mononucleosis (glandular fever), hepatitis and Cytomegalovirus infection
- other acute infections such as pertussis
- some protozoal infections, such as toxoplasmosis and American trypanosomiasis (Chagas disease)
- chronic intracellular bacterial infections such as tuberculosis or brucellosis
- chronic lymphocytic leukemia
- acute lymphoblastic leukemia
- post-splenectomy state
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.
- Miale, John B. Laboratory Medicine: hematology. 5th. St. Louis: C.V. Mosby, 1977.
- For example, if the total WBC count is 30,000, and the %lymphocytes is 30, the absolute lymphocyte count is 9,000 per microliter.
- 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.