Tumor infiltrating lymphocytes (TIL) therapy is an adoptive cellular therapy using a patient’s own TILs to target and destroy tumors. Developed by Dr Steven Rosenberg and his team at NIH in 1982, it involves extracting TILs from a tumor, expanding them with interleukin-2 (IL-2) in the lab, and reinfusing them into the patient. Initial studies using mouse models demonstrated that combining TILs with cyclophosphamide conditioning and IL-2 effectively treated liver and lung metastases. This laid the groundwork for applying TIL therapy to human cancers.