Lymph node

A lymph node is a small bean-shaped organ, from a few millimeters to a couple of centimeters long, placed along the lymphatic vessels that return tissue fluid to the blood. The body has several hundred of them, clustered especially in the neck, armpits, chest, abdomen and groin. Each node has a fibrous capsule. Several afferent lymphatic vessels bring lymph in around its edge, and an efferent vessel carries it out at the hilum, an indented side where blood vessels also enter and leave.

Inside, lymph trickles through channels called sinuses, lined by macrophages and dendritic cells that trap microbes and debris. The outer cortex holds B-cell follicles, whose germinal centers fill with dividing B cells during a response; the deeper paracortex is rich in T cells; and the central medulla contains cords of plasma cells and macrophages. Because lymphocytes keep circulating through the nodes, these act as meeting points where dendritic cells present antigens from nearby tissues and adaptive immune responses begin.

During an infection, the nodes draining the area swell and may feel tender as lymphocytes multiply. Many cancers spread first to regional lymph nodes, so doctors examine or remove them, for example the sentinel node, to stage the disease.