hematology and oncology
Lymphocytopenia
Introduction
Lymphocytopenia is a total lymphocyte count of<1000/mcL (1 × 109/L) in adults or<3000/mcL (<3 × 109/L) in children<2 years. Sequelae include opportunistic infections and an increased risk of malignant and autoimmune disorders. If the complete blood count reveals lymphocytopenia, testing for immunodeficiency and analysis of lymphocyte subpopulations should follow, usually after the patient has recovered from any acute event. Treatment is directed at the underlying disorder. Lymphocytes are components of the cellular immune system and include T cells, B cells, and natural killer (NK) cells, all of which are present in the peripheral blood; about 75% of the lymphocytes are T cells, 20% B cells, and 5% NK cells. Because lymphocytes account for only 20 to 40% of the total white blood cell (WBC) count, lymphocytopenia may go unnoticed if the WBC count is checked without a differential. The normal lymphocyte count in adults is 1000 to 4800/mcL (1 to 4.8 × 109/L; in children<2 years, the normal count is 3000 to 9500/mcL (3 to 9.5 × 109/L). At age 6 years, the lower limit of normal is 1500/mcL (1.5 × 109/L). Different laboratories may have slightly different normal values. Almost 65% of blood T cells are CD4+ (helper) T cells. Thus, most patients with lymphocytopenia have a reduced absolute number of T cells, particularly in the number of CD4+ T cells. The average number of CD4+ T cells in adult blood is 1100/mcL (range, 300 to 1300/mcL [1.1 × 109/L with a range of 0.3 to 1.3 × 109/L]), and the average number of cells of the other major T-cell subgroup, CD8+ (suppressor) T cells, is 600/mcL (range, 100 to 900/mcL). It is important to note that the lymphocytes in the blood represent only a small proportion of the total lymphocyte pool, and the lymphocyte count may not always correlate with the composition and numbers of lymphocytes in other lymphoid (eg, nodes, spleen) and non-lymphoid (eg, lung, liver) tissues. Similarly, deficiencies of particular subsets of lymphocytes (eg, CD8+, B, NK cells) can also be missed. 