Low lymphocytes in routine blood tests can be caused by infections or by the presence of certain aspects of the patient’s immune deficiency. When the body is invaded by external bacteria, the body will produce a lot of neutrophils to fight the bacteria, and the percentage of lymphocytes will decrease accordingly, and the white blood cell and neutrophil values in the routine blood test will increase, and the patient will also have some symptoms of infection, which is a condition of lower lymphocytes. Another condition is that the patient has immune diseases such as AIDS, because lymphocytes are immune cells in the body, and when the patient has immune defects caused by viruses, radiotherapy, chemotherapy and other factors, lymphocytes will be reduced accordingly, and then we should be alert and go to the hospital as soon as possible to avoid delaying treatment.