CorpusHuman Anatomy

Lymph Node — Sectioned

Bisected node showing lymph and lymphocyte traffic

A bean-shaped lymph node cut through its long axis, with the convex surface on the left and the hilum on the right. Lymph (green particles) enters through several valved afferent vessels, spreads through the subcapsular sinus, runs inwards along the trabecular sinuses to the medullary sinuses, and leaves through a single efferent vessel at the hilum. Along the way, macrophages filter it and antigen-presenting cells sample it.

The parenchyma is arranged in zones. The outer cortex holds B-cell follicles (primary follicles are uniform; secondary follicles have a pale germinal centre under a darker mantle cap). The paracortex is the T-cell zone with dendritic cells. The medulla has plasma-cell-rich medullary cords between the sinuses. Blood lymphocytes reach the node from the hilar artery and cross the cuboidal endothelium of the high endothelial venules (animated) into the paracortex.

Step-by-step

  1. Afferent lymph arrives

    Several afferent lymphatic vessels pierce the convex capsule. Their valves keep flow one-way. They bring interstitial fluid, free antigen, migrating dendritic cells and some lymphocytes from an upstream territory.

  2. Subcapsular sinus

    Lymph first spreads through the subcapsular (marginal) sinus beneath the capsule. Sinus macrophages capture particulate antigen and pass it to B cells in the follicles. Small soluble antigens enter conduits that run into the cortex.

  3. Cortex: B-cell follicles

    The outer cortex contains B-cell follicles on a network of follicular dendritic cells. Primary follicles hold resting naive B cells. Once antigen and T-cell help arrive, they become secondary follicles.

  4. Germinal centres

    In secondary follicles, activated B cells proliferate (dark zone) and undergo somatic hypermutation. They are then selected for high-affinity binding with help from Tfh cells (light zone). The result is class-switched plasma cells and memory B cells.

  5. Paracortex and HEVs

    The paracortex is the T-cell zone. Naive lymphocytes leave the blood across the high endothelial venules (L-selectin–PNAd, CCR7–CCL21, LFA-1–ICAM-1) and scan dendritic cells for their specific antigen. If they find none, they leave in the efferent lymph within about a day.

  6. Trabecular and medullary sinuses

    Lymph runs along the trabecular sinuses into the medullary sinuses, which are lined by macrophages that phagocytose microbes, debris and tumour cells. This filtering is why nodes enlarge, and can harbour metastases, when their territory is diseased.

  7. Medullary cords

    Plasma cells collect in the medullary cords between the sinuses and secrete antibody into the outflowing lymph.

  8. Leaving at the hilum

    The sinuses converge at the hilum into the efferent lymphatic vessel, which carries lymph enriched with lymphocytes and antibody onward. The nodal artery enters and the vein leaves at the same hilum.

Labelled parts