Merck Manual

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Overview of Leukopenias

By

David C. Dale

, MD, University of Washington

Reviewed/Revised Apr 2023
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Leukopenia is a reduction in the circulating white blood cell (WBC) count to < 4000/mcL (< 4 × 109/L). It is usually the consequence of a reduced number of circulating neutrophils, although a reduced number of lymphocytes, monocytes, eosinophils, or basophils may also contribute. Thus, immune function can be generally decreased.

Neutropenia Neutropenia Neutropenia is a reduction in the blood neutrophil count. If it is severe, the risk and severity of bacterial and fungal infections increase. Focal symptoms of infection may be muted, but fever... read more is a reduction in blood neutrophil count to < 1500/mcL (< 1.5 × 109/L). People of African or Middle Eastern descent often have neutrophil counts as low as 500/mcL (0.5 × 109/L), which is referred to as ethnic neutropenia. This difference is now usually attributable to polymorphisms in the Duffy antigen receptor-1 gene (DARC). Neutropenia accompanied by monocytopenia and lymphocytopenia causes more severe immune deficits than neutropenia alone.

Lymphocytopenia Lymphocytopenia 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... read more , in which the total number of lymphocytes is < 1000/mcL (< 1 × 109/L) in adults, is not always recognized as a decrease in the total WBC count because lymphocytes account for only 20 to 40% of the total WBC count. The consequences of the lymphopenia can depend on the lymphocyte subpopulation(s) that are decreased.

Monocytopenia Monocytopenia Monocytopenia is a reduction in blood monocyte count to < 500/mcL (< 0.5 × 109/L). Risk of certain infections is increased. It is diagnosed by complete blood count with differential... read more is a reduction in blood monocyte count to < 500/mcL (< 0.5 × 109/L). Monocytes migrate into the tissues where they become macrophages, with specific characteristics depending on their tissue localization.

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