a | Cancer cells in primary tumours are surrounded by a complex microenvironment comprising numerous cells including endothelial cells of the blood and lymphatic circulation, stromal fibroblasts and a variety of bone marrow-derived cells (BMDCs) including macrophages, myeloid-derived suppressor cells (MDSCs), TIE2-expressing monocytes (TEMs) and mesenchymal stem cells (MSCs). b | Invasive human breast cancer stained with haematoxylin and eosin, in which a prominent infiltration of leukocytes (indicated by white arrows) is evident at the invasive margin. c | Macrophages at the invasive edge of pancreatic islet cancers express cathepsin B (green), which is associated with loss of epithelial cadherin (red) on the neighbouring cancer cells. Cell nuclei are visualized by DAPI (blue). Part c reproduced, with permission, from REF. 151 © (2006) Cold Spring Harbor Laboratory Press.