Leukopenia (from Greek λευκός (leukos), meaning 'white', and πενία (penia), meaning 'deficiency') is a decrease in the number of leukocytes. Found in the blood, they are the white blood cells, and are the body's primary defense against infection. Thus leukopenia places individuals at increased risk of infection.
Neutropenia, a subtype of leukopenia, refers to a decrease in the number of circulating 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. This should not be confused with agranulocytosis.