Merck Manual

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Overview of Leukopenias


Mary Territo

, MD, David Geffen School of Medicine at UCLA

Last review/revision Aug 2021 | Modified Sep 2022
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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 characterized by 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) in White people and < 1200/mcL (< 1.2 × 109/L) in Black people. It is sometimes accompanied by monocytopenia and lymphocytopenia, which cause additional immune deficits.

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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