Lymph node
Small bean-shaped filters along the lymph vessels where immune cells gather. They are often the first place a cancer spreads, so surgeons check them to see how far the disease has gone.
Lymph drains fluid from tissues back to the blood, passing through nodes where T and B cells inspect it for infection; cancer cells that enter lymph vessels lodge in the nearest nodes first, so node involvement (the N in TNM) is a key marker of a cancer's tendency to spread and raises the stage. Sentinel-node biopsy identifies and removes only the first node or two that drain a tumour, sparing patients the arm swelling (lymphoedema) that removing all nodes causes. Lymph nodes are also where immune responses against a tumour are launched, and where immunotherapy needs functioning T cells to be primed.