The normal range for peripheral blood leukocyte count in adults is (3.5-9.5) × 109/L. A leukocyte count is considered above normal when it is consistently higher than 9.5 × 109/L, so a leukocyte count of (22-26) × 109/L is considered leukocytosis. Similarly, leukocytes contain multiple cellular components, and it is necessary to break down exactly which of these cellular components are increased in order to make a targeted differential diagnosis. For example, if an absolute increase in lymphocytes is predominant, the focus should be on chronic lymphocytic leukemia, and when eosinophils are significantly increased, a detailed examination for a disease such as eosinophilia needs to be considered. This section focuses on the analysis of the causes of neutrophilia, the most common of which are the following: Neutrophils increase in various infectious diseases, such as streptococcal pneumonia and parasitic infections. When myocardial infarction, surgery and other necrotic diseases, as well as rheumatoid and other immune system diseases occur, the immune system in the patient’s body will play a protective role to remove harmful substances from the body, thus causing the neutrophil count to rise to varying degrees. In this case, it is only necessary to carry out relevant treatment for the cause of the disease, and the increased neutrophils will gradually return to normal. However, when hematologic malignant diseases such as leukemia occur, the leukocyte count in the patient’s body can increase significantly, up to tens of times the normal level. If the leukocyte count in the blood exceeds 100×109/L, the patient may experience leukocyte stasis, with dangerous symptoms such as respiratory distress, hypoxemia, unresponsiveness, slurred speech and intracranial hemorrhage. Therefore, when the leukocyte count in the blood exceeds 100×109/L, emergency treatment measures such as blood cell separators can be taken to prevent complications from arising, while targeted treatment should be given for the primary disease, such as combination chemotherapy for acute myeloid leukemia, in order to finally control the disease. Therefore, a leukocyte of (22-26) × 109/L is considered leukocytosis, and the severity is related to the etiology behind it, rather than being determined by looking at the cell count in isolation. Patients should seek prompt medical attention. The cell type of their increase needs to be broken down and the etiology behind it needs to be clarified for accurate and targeted treatment.