Leukopenia

An abnormally low number of circulating white blood cells below the lower limit of the normal reference range (adults: 4-11 × 10⁹/L), typically reflecting reduced leukocyte production, increased destruction, or sequestration, and associated with increased susceptibility to infection. Possible causes are chemotherapy, radiation therapy, viral infections (EBV, HIV/AIDS, hepatitis), bone marrow disorders (leukemia, aplastic anemia, myelofibrosis, myelodysplastic syndrome), autoimmune disease (SLE, rheumatoid arthritis), nutritional deficiency (vitamin B12, folate, copper), medications (antibiotics, antipsychotics, immunosuppressants), and hypersplenism.