TY - CHAP M1 - Book, Section TI - Lymphocytosis and Lymphocytopenia A1 - Vasu, Sumithira A1 - Caligiuri, Michael A. A2 - Kaushansky, Kenneth A2 - Lichtman, Marshall A. A2 - Prchal, Josef T. A2 - Levi, Marcel M. A2 - Press, Oliver W. A2 - Burns, Linda J. A2 - Caligiuri, Michael PY - 2015 T2 - Williams Hematology, 9e AB - SUMMARYLymphocytosis is defined as an absolute lymphocyte count exceeding 4 × 109/L, whereas lymphocytopenia is defined as a total lymphocyte count less than 1.0 × 109/L. Lymphocytosis can be categorized as either polyclonal or monoclonal. Monoclonal lymphocytosis reflects an underlying clonal lymphoid disease in which the numbers of lymphocytes are increased because of the acquisition of somatic mutations resulting in clonal expansion of a lymphocyte progenitor. This expansion can be stable, such as monoclonal B-cell lymphocytosis or a progressive malignancy such as acute lymphocytic leukemia, whereas polyclonal lymphocytosis is most commonly the result of stimulation or a reaction to factors extrinsic to lymphocytes, generally infections and/or inflammation. Lymphocytopenia, on the other hand, typically reflects depletion of T cells, the most abundant lymphocyte subtype in the blood. The most common cause of such T-cell depletion is a viral infection, such as infection with the human immunodeficiency virus, although other causes exist. This chapter outlines the conditions associated with abnormalities in the numbers of circulating lymphocytes in the blood. It also serves as a useful road map to other chapters in the book that describe in detail those conditions that commonly are associated with abnormalities in the absolute numbers of circulating lymphocytes. SN - PB - McGraw-Hill Education CY - New York, NY Y2 - 2024/03/28 UR - accessmedicine.mhmedical.com/content.aspx?aid=1121097369 ER -