Investigators have identified a new class of human immune cells that behave like stem cells. These cells, a subtype of T lymphocytes, which comprise a small fraction of white blood cells, may prove more effective than any previously reported type of T cell for treating tumors. The study, by scientists at the National Cancer Institute (NCI), part of the National Institute of Health, describes how these stem cell-like T cells can trigger a prolonged immune attack against tumor cells by continuously generating killer T cells and regenerating themselves. The findings were published online Sept. 18, 2011, in Nature Medicine.