Leukocytes is a general term for a group of cells in the blood that includes mainly monocytes, lymphocytes, and neutrophils. The normal value of leukocytes is (4-10) × 109/L, and above this range is called leukocytosis. The leukocytosis found after laboratory tests can be physiological leukocytosis and pathological leukocytosis. Physiological leukocytosis is temporary, and babies usually have no obvious accompanying symptoms, and their mental and dietary status are normal. The normal leukocyte values are different at different ages, and in newborns and infants the leukocytes can be higher than the regular values, but of course there is a normal range, and too high is not allowed. Pathological leukocytosis can be seen in a variety of diseases: such as various infections, which can be local or systemic; non-infectious inflammatory diseases: such as rheumatic diseases; blood disorders, such as acute granulocytic leukemia; tissue necrosis, such as surgical trauma and burns; and also such as acute blood loss and hemolysis, malignant tumors, metabolic disorders, drugs and toxins, all of which can appear as leukocytosis. The increase in leukocytes itself is not the cause of the damage done to the body, but rather the problem is to find out the cause of the elevated leukocytes. Since there are many clinical causes of leukocyte elevation and different clinical manifestations, it is not possible to make a generalization. Detailed medical history, physical examination, laboratory tests, and medical imaging are needed to determine the causative factors and to treat them.