The causes of low blood white blood cells include infectious diseases such as viral hepatitis and chickenpox, autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis, and hematologic diseases such as leukemia. Leukocyte is an important nucleated blood cell in the human body, which has important physiological significance. The level of leukocyte in normal population should be between (4.0 to 10.0) x 10^9/L, and lower than that range is called leukopenia. A common reason for leukopenia is that the patient is suffering from viral infections such as viral hepatitis and chickenpox, which cause rapid depletion of neutrophils and production of negative hematopoietic regulatory factors, leading to a decrease in the overall leukocyte level. In addition to the above, autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis will destroy a large number of white blood cells in the patient’s body. A variety of blood diseases, such as aplastic anemia, acute leukemia, etc., will affect the hematopoietic function of the bone marrow, reduce the proliferation of normal hematopoietic cells in the bone marrow, resulting in a decrease in the number of newly synthesized leukocytes in the body, etc., which is also a common cause of low leukocyte levels in the patient’s blood count. To summarize, there are many reasons for patients to have low white blood cells in routine blood tests, and it is recommended that patients undergo further examination to identify the specific causes and actively treat them.