There are five reasons for the decrease in white blood cells: first, infections, commonly caused by viral infections such as influenza, neoconiosis, viral hepatitis, rubella, cytomegalovirus infection, etc. Bacterial infections are common in typhoid fever, paratyphoid fever, and certain protozoal infections such as malaria, black fever, etc. Second, blood system diseases, such as aplastic anemia, hemophagocytic syndrome, severe iron deficiency anemia, etc., can cause a decrease in white blood cells and other blood cells. Third, physicochemical factors, such as radionuclides and X-rays, can cause a decrease in leukocytes due to physical factors. Fourth, the hyperfunction of the mononuclear macrophage system and the enlargement of the spleen caused by various reasons can cause the spleen to enhance the phagocytosis of leukocytes and platelets, thus causing a decrease in the total number of leukocytes. Fifth, autoimmune diseases, such as systemic lupus erythematosus, produce autoantibodies leading to neutrophil and leukocyte reduction.