abnormally low number of white blood cells (leukocytes (leukocyte)) in the blood circulation, defined as less than 5,000 leukocytes per cubic millimetre of blood. Leukopenia often accompanies certain infections (infectious disease), especially those caused by viruses or protozoans. Other causes of the condition include the administration of certain drugs (e.g., analgesics, antihistamines, and anticonvulsants), debilitation, malnutrition, chronic anemias (anemia), some spleen disorders, agranulocytosis, lupus erythematosus, and anaphylactic shock (anaphylaxis).