CCL21 and cancer: Whilst lymph node spread of cancer has been known for decades, more recent evidence has implicated the lymphatics not simply as passive highways for tumour cell spread but also as facilitators in many other processes, including the active recruitment of tumour cells to local and distal lymph nodes162 through mechanisms such as CCL21–CCR7 signalling163,164, promoting the survival of metastasising cancer stem cells via CXCL12–CXCR4 signalling165,166, and modulating the host inflammatory response to alter tumour immune surveillance167–170.