Low lymphocyte counts in radiation therapy may be due to bone marrow suppression caused by radiation therapy or due to the use of drugs such as glucocorticoids during treatment. Lymphocytes are the core of the body’s immune response, and their differentiation into T-lymphocytes, B-lymphocytes, natural killer cells and other cells are involved in a variety of immune responses. X-rays and gamma rays used in radiation therapy inhibit the proliferation and differentiation of bone marrow hematopoietic stem cells while inhibiting tumor growth and division, resulting in lower lymphocyte counts. In addition, if glucocorticosteroids such as prednisone are used to treat severe inflammation and high fever during treatment, they will also inhibit the proliferation and differentiation of white blood cells, resulting in a decrease in lymphocyte count. When this phenomenon occurs, it is recommended to go to the hospital in time.