I was wondering what are causes of low lymphocytes?

Many: You know already that the reference ranges given for labs are set to that several percent of healthy folks fall outside on any particular test. The stresses of daily living cause the absolute lymphocyte and granulocyte counts to fluctuate wildly. If you're quite low and there's no explanation, think about asking for a HIV check. Best wishes.

See below: Common causes are: infectious diseases, such as aids, viral hepatitis, tuberculosis, and typhoid fever. Autoimmune disorders, such as lupus. (autoimmune disorders occur if the body’s immune system mistakenly attacks the body’s cells and tissues.) steroid therapy. Blood cancer and other blood diseases, such as hodgkin's disease and aplastic anemia. Radiation and chemotherapy (treatments for cancer).