White blood cells in the blood are the body’s patrols against microbial invasion. When a foreign body such as a microorganism invades, white blood cells enter the invaded area and surround, devour, and destroy the invading microorganism, which is why white blood cells are called the “white guards” of the body. A leukocyte count of <4×109/L in adults, <4.5×109/L in children 10-12 years old, and <5×109/L in children younger than 10 years old is called leukopenia, and an absolute granulocyte count of <0.5×109/L is called granulocyte deficiency. Normal human leukocyte values fluctuate to varying degrees at different times of the day, and sometimes the highest values differ by nearly two times from the lowest values. During the year, leukocyte values are higher in the winter time than in the summer, and they increase during physical activity and pregnancy. Therefore, for people with leukocyte values at critical levels, it is important to test at roughly constant times and to exclude interference from other physiological factors. It is necessary to combine the results of several tests to make a judgment.