TILs, or tumor-infiltrating lymphocytes, are the specialized T cells from the immune system that are found in tumors. When activated, TILs have the ability to recognize and kill cancer cells. But cancer can suppress TILs. In TIL therapy, a form of immunotherapy, doctors remove a tumor from a patient, identify the fighter T cells, grow them in a lab, then infuse them back into the patient to fight the cancer.