CD4 and neoplasm: In brief, tumor-derived Treg cells develop locally within or in the vicinity of the cancer focus, to which they are drawn as naïve CD4+ T cells by tumor-derived cytokines, and they show higher suppressive activity than physiological Treg cells, thereby suppressing other immune cells’ activity through the production of massive amounts of IL-10 (among others) or directly and indirectly killing them via Fas/FasL- or CD40/CD40L-mediated cell death or via enzymatic starvation [14,15,38,39,40,41,42].