Illustration showing a tumour microenvironment (TME), the complex cellular domain surrounding and supplying a tumour. A TME consists of cancer cells, extracellular matrix (thin, web-like strands), new blood vessels (dark orange, branching), cancer-associated fibroblasts, and a variety of immune cells such as macrophages, T cells, dendritic cells, neutrophils, and natural killer (NK) cells. Tumours are not just clusters of malignant cells; in many cases, cancer cells make up less than half of the TMEs total mass. The new blood vessels that form are particularly significant; they supply the tumour with oxygen and nutrients, and provide a route for cancer cells to potentially metastasize (spread) to other parts of the body.