A type of immune cell that has moved from the blood into a tumor cell. Tumor infiltrating lymphocytes are thought to be a sign that the immune system is trying to attack the cancer. In cancer therapy, tumor infiltrating lymphocytes are removed from a patient’s tumor, then treated in the laboratory with substances that make them grow and turn into cells that can kill the patient’s cancer cells. Large numbers of these activated lymphocytes are then reinfused into the patient to help the immune system fight cancer. Also called TIL.