When the white blood cell count in peripheral blood is below 4.0 times 10 to the ninth power per liter, it is termed leukopenia. According to the cause, it can be divided into the following three aspects: First, disturbance in the production of white blood cells, including reduced proliferation of stem cells or regeneration disorders. Second, excessive destruction of white blood cells, such as from infections or immunological factors, leading to increased destruction of white blood cells. Third, abnormal distribution of white blood cells, due to various reasons, causes an increase in white blood cells in the marginal pool and a decrease in the circulating pool, which can also lead to leukopenia.