Low white blood cells are seen in infections, hematologic disorders, physicochemical injuries, hyperfunction of monocyte-macrophage system, and autoimmune diseases. 1. Infections: typhoid fever, paratyphoid infection, or viral infectious diseases such as influenza, viral hepatitis, chickenpox, rubella, cytomegalovirus infection, and protozoal infections such as malaria and black fever. 2. Blood system diseases: such as severe iron deficiency anemia, paroxysmal sleep hemoglobinuria, aplastic anemia and so on. 3. Physical and chemical damage: physical factors such as X-rays, radionuclides, etc. Chemical factors can be seen in lead and benzene poisoning, as well as chloramphenicol and sulfonamides. 4. Monocyte-macrophage system hyperfunction: some lymphomas, portal cirrhosis, etc. 5. Autoimmune diseases: connective tissue type of autoimmune diseases, most often seen in leukopenia, such as systemic lupus erythematosus. When the white blood cell decrease occurs, you should consult a doctor in time to clarify the cause of the disease through further examination, and actively take targeted treatment under the guidance of a professional doctor, so as not to delay the course of the disease.