Lymph nodes slow the flow of lymph on purpose so immune cells can react
An adult body holds about 450 lymph nodes, bean-shaped filters averaging 15 millimetres long. Lymph pours into each one through many vessels but usually leaves through a single exit. That bottleneck is deliberate: slowing the stream buys time for immune cells to meet invaders and multiply before trouble spreads.
A node is wrapped in a fibrous capsule whose inner struts, the trabeculae, reach toward its centre. Inside lies a crowded outer cortex and a sparser inner medulla. Lymph enters across the rounded surface, trickles through a chain of channels beneath the capsule, through the cortex and into the medulla, then exits at the hilum, the dimple where blood vessels also come and go. The channels are too narrow for resident macrophages to escape, keeping those scavengers at their posts.
The layout is organised by job. B cells, which make antibodies, cluster in follicles in the outer cortex; T cells and dendritic cells gather in the deeper paracortex; and antibody-secreting plasma cells sit in cords in the medulla. A mesh of reticular fibres holds everything together and doubles as a molecular sieve that keeps pathogens from slipping back into the bloodstream. Dendritic cells carry fragments of microbes in from the tissues and show them to T cells, and a T cell with the matching receptor switches on. B cells grab their targets straight from incoming lymph.
Nodes concentrate in the trunk, near major structures, though some sit close to the surface under the arm, in the neck and near the groin, where they can be felt when enlarged. Swelling can reflect anything from a mild sore throat to cancer, and the state of nearby nodes is central to how cancers are staged. For all their filtering, though, nodes play no part in detoxification.
The brain has none; its lymphatic vessels drain to nodes deep in the neck, yet nerves from the central nervous system still help regulate what nodes do. Pigs, rhinos, hippos and some whales have inverted nodes that run the flow from centre to edge, apparently working just as well.
Source: Lymph node
