Leukocytes are mainly composed of neutrophils, monocytes, eosinophils, basophils and lymphocytes, so there are two indicators of the percentage and absolute value of these cells in a routine blood test. It is easy to understand that an abnormal percentage of a certain cell can be abnormal by itself or it can be a relative change due to abnormalities in other cellular components, so only the absolute value can be used to determine whether a certain cellular component is increasing and decreasing. A peripheral blood leukocyte count that is consistently below 3.5 x 109/L is considered below normal and is referred to as leukopenia, the most common of which is neutropenia. According to the degree of reduction, neutropenia can be divided into mild ≥1.0×109/L, moderate (0.5-1.0)×109/L and severe <0.5×109/L. Severe neutropenia is also known as neutrophil deficiency, or "granulocytosis". Therefore, 2.8×109/L leukocytes cannot determine the severity of neutropenia, but the absolute value of neutrophils must be known to determine it. Patients with mild neutropenia may not have specific clinical symptoms, or may only have mild discomfort such as low fever and malaise. Moderately and severely reduced patients are prone to infections and non-specific symptoms such as fatigue, weakness, dizziness, loss of appetite, etc. The common sites of infection are the respiratory tract, gastrointestinal tract and genitourinary tract, where headache, high fever, mucosal necrosis and hemorrhagic ulcers may occur, and even severe sepsis or infectious shock, which may be life-threatening. Therefore, patients with granulocytopenia need to seek immediate medical attention once they become febrile. Neutropenia has many causes and is not a single disease, and the cause needs to be clearly identified before targeted treatment can be given. Common causes are as follows: certain viral infections or sepsis can cause excessive depletion of leukocytes at the site of inflammation; autoimmune system diseases such as hypersplenism, certain hepatitis or systemic lupus erythematosus can also destroy leukocytes in the body, resulting in reduced numbers; diseases affecting human hematopoietic stem cells, such as aplastic anemia, myelodysplastic syndrome, bone marrow metastatic cancer and other neoplastic diseases can Diseases affecting human hematopoietic stem cells, such as aplastic anemia, myelodysplastic syndrome, bone marrow metastatic cancer and other neoplastic diseases, can affect the production and maturation of leukocytes, resulting in a decrease in the number of leukocytes. In addition, abnormal distribution of neutrophils in the body can also lead to a decrease in their absolute value, a condition that does not cause obvious symptoms. Therefore, when leukocytes are 2.8×109/L, it is necessary to know the absolute value of neutrophils. If neutrophils are mildly reduced, no special protection is needed, but the number of leukocytes should be monitored regularly, and if they are consistently lower than normal, systematic and formal examinations and laboratory tests should be improved in time to clarify the diagnosis and then treat accurately.