The fact that tumors are composed of both tumor cells and host cells has long been known. These tumor-associated cells include vascular endothelial cells and pe- cytes, as well as inflammatory cells such as neutrophils, monocytes, macrophages, mast cells and eosinophils, and lymphocytes. The tumor cells also interact with stromal cells and with elements of the tissue extracellular matrix. What has been less appreciated is the role that these cells...