If there is a decrease in white blood cells, we mainly look at the total number of white blood cells and the absolute value of neutrophils in our routine blood test. The causes of leukopenia are inseparable from neutrophils, and can be mainly divided into three categories. The first category is a production defect of neutrophils, the second is excessive destruction and consumption of neutrophils, and the third is abnormal distribution of neutrophils. A major cause of decreased production of white cells and neutrophils is damage to hematopoietic stem cells by ionizing chemicals. Additionally, immune reactions and bone marrow diseases can also lead to decreased production. Another cause is a production defect due to maturation disorders in neutrophils within white blood cells, leading to reduced white blood cell production. Maturation disorders are mainly due to a lack of raw materials needed for white blood cell production. Excessive destruction and consumption of white blood cells are often due to immune factors. Immune factors are common in autoimmune diseases, thus such diseases can also lead to leukopenia. The last category, abnormal distribution, can be a severe infection that causes an abnormal distribution of white cells, retaining them alongside other substances, such as in patients with enlarged spleens, which can provoke leukopenia by causing granulocytes to be retained in circulation, especially in areas like the spleen.