Lymphocytopenia

A decrease in circulating lymphocytes below the reference range (1.0-4.8 × 10⁹/L, children <2 years: 3.0-9.5 × 10⁹/L), potentially impairing cell-mediated and humoral immune responses and predisposing to infection and immune dysregulation. Possible causes are viral infections (HIV/AIDS, COVID-19, influenza, viral hepatitis, measles), severe infections (tuberculosis, sepsis, pneumonia), medications (corticosteroids, chemotherapy, radiation therapy, immunosuppressants), malnutrition, nutritional deficiency (zinc, folic acid, or vitamin B12), autoimmune disorders (SLE, rheumatoid arthritis, Sjögren's syndrome, sarcoidosis), malignancy (Hodgkin lymphoma, non-Hodgkin lymphoma, and leukemia), and genetic diseases (SCID, DiGeorge syndrome, or Wiskott-Aldrich syndrome).