Tumor-infiltrating lymphocytes are white blood cells that have left the blood and are embedded in the tumor. Tumor mass comprises tumor cells in addition to stromal cells that enable tumors to grow and expand, endothelial cells, leukocytes, fibroblasts and a complex extracellular matrix (ECM). Together these cells form the tumor microenvironment (TME).
TILs are polymorphic and contain various populations of immune cells such as T cells, NK cells, dendritic cells and macrophages. TILs directly surround the tumors within the tumor stroma.