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 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, 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 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.
