LEUKOPENIA

n. abnormal decrease in the number of white blood cells in the blood

Leukopenia (or leukocytopenia, or leucopenia or leukopaenia) is 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.
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Leukopenia /leu·ko·pe·nia/ reduction of the number of leukocytes in the blood below about 5000 per cubic mm. [more]Leukopenia - Community and Resources

Noun
1. an abnormal lowering of the white blood cell count
(synonym) leucopenia
(hypernym) blood disease, blood disorder
(hyponym) neutropenia

Shortage of white blood cells.