The normal absolute value of lymphocytes is (0.8~4)×10^9/L. If it is lower than the lower limit of the reference value, it is called low absolute value of lymphocytes, and the causes include: congenital factors, acquired factors, and medical factors. 1. Congenital factors: including hypogammaglobulinemia, congenital thymic dysplasia and other congenital immunodeficiency diseases; 2. Acquired factors: such as AIDS, HIV virus attacks lymphocytes, the absolute value of lymphocytes in AIDS patients will decrease; severe bacterial infections, the absolute value of neutrophils will be significantly increased with the absolute value of lymphocytes decreased; 3. Medical factors: the application of cytotoxic drugs, glucocorticoids or radiation therapy will also affect bone marrow hematopoiesis, resulting in a decrease in the absolute value of lymphocytes. Patients with low absolute lymphocyte values are advised to visit the hematology department of the hospital for further examination, active identification of the cause of the disease and active treatment.