The evidence seems to be growing in favor of the stem cell theory of cancer with the emergence of studies demonstrating the
parallel mechanisms of self renewing pathways in stem cells and particular subsets of cancer cells. The finding of leukemia stem cells
and subsequently breast cancer stem cells (BCSC) further supports the concept. The importance of these findings lends itself to the selfrenewal
properties of stem cells in addition to the survival characteristics of stem cells, mechanisms that will have to be overcome when
creating treatment modalities. In particular, research has shown that stem cells and a specific type of stem cells, mesenchymal stem cells
(MSC), have special drug effluxing properties and some interactions with particular cells of the immune system that may serve major
roles in immunosuppresion and overall tumor cell survival. Furthermore, the recent discovery of epithelial to mesenchymal transition
(EMT) has laid out a possible mechanism for tumor cells to lose particular phenotypic epithelial cell markers and gain phenotypic mesenchymal
cell markers. This process is implicated in metastasis in addition to overall tumor survival and evasion of the immune system.
This review examines the current understanding of how tumor cells evade the immune system, but will first explore stem cells, cancer
stem cells, normal immune interaction with tumor cells, and EMT.