Leukopenia or leukocytopenia refers to a decrease in the number of circulating white blood cells (leukocytes) in the blood. As the principal function of white cells is to combat infection, a decrease in the number of these cells can place patients at increased risk for infection. In pancytopenia, all cell types in the blood (red blood cells and platelets) are similarly affected.

Neutropenia is the most common type of leucopenia, refers to the absence of neutrophil granulocytes, the most abundant white blood cells. The terms leukopenia and neutropenia may occasionally be used interchangeably, as the neutrophil count is the most important indicator of infection risk. However, neutropenia is more properly considered a subset of leukopenia as a whole.

Low white cell counts are associated with chemotherapy, leukemia (as malignant cells overwhelm the bone marrow), myelofibrosis and aplastic anemia (destruction of the bone marrow by the immune system). In addition, many common medications can cause leukopenia (caption above: Here the neutrophils are clumped together and cannot function properly as a result of a chemotherapy treatment).