Leukopenia is defined as a condition in which the white blood cell count in peripheral blood persistently remains below 4 times 10 to the ninth power per liter.

Since the components of white blood cells are mainly neutrophils and lymphocytes, particularly dominated by neutrophils, leukopenia is mostly caused by a decrease in neutrophils. The causes of leukopenia can be categorized into primary and secondary types. Primary leukopenia refers to cases where the cause is unknown, while secondary leukopenia could be due to acute infections, physical or chemical factors, or certain diseases such as hematologic disorders, connective tissue diseases, allergic diseases, or genetic disorders. In addition, there are also some acquired or idiopathic cases of granulocytopenia that fall under the category of leukopenia. Typical symptoms of leukopenia include dizziness, fatigue, muscle weakness, reduced appetite, and mental sluggishness.