Leukocytes are colorless, spherical, nucleated blood cells. The normal range of leukocytes in children is (5-12)×10^4/L. Leukocytes are not a homogeneous group of cells, but can be divided into three main categories: granulocytes, monocytes and lymphocytes, of which granulocytes can be divided into neutrophils, eosinophils and basophils. Leukocytes are the body’s “defenders” in the fight against disease. When germs invade the body, leukocytes pass through the capillary walls by deformation and concentrate on the invasion site to surround and engulf the germs. If the number of white blood cells in the body is higher than normal, it is likely that the body has an inflammatory condition. The most common cause of elevated white blood cells is infection, especially bacterial infections, and the degree of infection is often proportional to the increase in the number of white blood cells. Most infections in children are respiratory or digestive tract infections. Check your child’s respiratory tract, such as whether the nose is clear, sore throat, cough, etc., and the digestive tract to see if the baby has abdominal pain, anorexia, constipation or diarrhea, bloating, etc. Another cause of significantly elevated white blood cell count that requires special vigilance is hematopoietic disorders, when the white blood cells may increase to several or tens of times more than normal, requiring further blood smears, bone marrow aspiration and molecular biology and other tests to clarify the diagnosis. In addition, elevated white blood cell counts may be seen with the application of glucocorticoids. All of the above conditions that cause elevated white blood cells require prompt treatment.