A low lymphocyte percentage of 17.5% can be seen in both physiologic and pathologic decreases (e.g., immune system disorders, drug factors, etc.). The normal value of lymphocytes is 20% to 40%, and a lymphocyte percentage of 17.5% indicates a decrease. The common causes are as follows: 1. Physiological decrease: some people will have low lymphocyte percentage after strenuous exercise, which is a physiological decrease. 2. Pathological decrease (1) Immune system diseases: such as congenital immunodeficiency syndrome, gammaglobulin deficiency, AIDS, systemic lupus erythematosus and so on. (2) Drug factors: such as prolonged or massive application of chemotherapeutic drugs, such as cyclophosphamide, paclitaxel, vincristine, anti-lymphocyte globulin, etc., which will cause damage to normal cells when killing tumor cells, and also cause low percentage of lymphocytes. (3) Infectious diseases: such as severe suppurative infections can also cause a relative decrease in the percentage of lymphocytes. In addition to the above reasons, long-term exposure to radiation or after radiation therapy can also cause the above symptoms. It is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment.