The white blood cells in the blood are the body’s patrols against bacterial invasion. When bacteria and other foreign objects invade, white blood cells enter the invaded area and surround, devour and destroy the bacteria, so white blood cells are called the “white guards” of the body. As you can see, a decrease in the number of white blood cells will weaken the body’s antibacterial ability and make it susceptible to infection. The normal human white blood cell count is 4.0-10.0×109/L. The number of white blood cells in the blood varies widely at different times of the day and under different functional states of the body. When the leukocyte count in the peripheral blood is consistently below 4.0×109/L or less it is called leukopenia. Since the components of leukocytes are mainly neutrophils and lymphocytes, especially neutrophils, in most cases, leukopenia is caused by a decrease in neutrophils. When the neutrophil count is below 2,0×109/L, it is called neutropenia, and when the neutrophil count is below 0.5×109/L, it is called neutrophil deficiency. In the case of neutropenia, the onset of the disease is slow and the symptoms are mild, mainly malaise, dizziness, easy to catch a cold, often accompanied by low fever, pharyngitis, loss of appetite, limb weakness, insomnia, palpitations, chills and lumbago, etc. In the case of granulocyte deficiency, the onset of the disease is rapid, with sudden chills or chills, high fever, headache, arthralgia, extreme malaise, and in severe cases, dysphagia, delirium or coma. There are various causes of leukopenia, the main causes are: 1, reduced leukocyte production: ionizing radiation such as X-rays, chemical toxins such as benzene, anti-cancer drugs, etc. can inhibit the bone marrow; bone marrow hematopoietic stem cell diseases such as aplastic anemia, or leukemia, myeloma, metastatic tumor cells infiltrate the bone marrow hematopoietic tissue, can affect leukocyte production. 2, leukocyte maturation disorders: such as folic acid and vitamin B12 deficiency or metabolic disorders, myelodysplastic syndrome (MDS) and other causes of hematopoietic cell differentiation and maturation disorders. 3, excessive destruction of leukocytes: a variety of drugs such as antipyretics, antituberculosis drugs, antithyroid drugs, antiepileptic drugs, etc. can be specific immune response to lead to the destruction of leukocytes, as well as autoimmune diseases (such as systemic lupus erythematosus, rheumatoid arthritis, etc.) through the immune mechanism leading to leukopenia, and various infections such as viral infections or sepsis, in the peripheral blood and inflammatory sites consumed too much. 4. Abnormal leukocyte distribution: Abnormalities in the distribution of leukocytes caused by attachment to the vessel wall and migration to the spleen, such as various allergies and endotoxemia, as well as hypersplenism. The etiology of leukopenia is complex, so the following conditions should be given high priority: 1. Combination of other blood cell abnormalities, such as anemia and thrombocytopenia. 2. Abnormal blood cells, such as naïve or primitive cells, are present in the blood. 3.Neutrophils less than 1.5×10^9/liter or combined with infection. 4. Leukopenia lasts for more than 4 weeks. In conclusion, patients with leukopenia should avoid going to public places as much as possible to prevent whistle infection; avoid taking drugs that cause bone marrow damage or leukopenia; avoid raw, cold and unclean diet to avoid digestive system infection. Also, the cause of the disease should be diagnosed as soon as possible and treated promptly.