Lymph node

Overview

Location

Along lymphatic vessels throughout the body; superficial groups in the neck, axillae and groins; deep groups in the mediastinum, along the aorta and iliac vessels, and in the mesenteries.

Parts & subdivisions

1. Capsule and trabeculae
   Dense irregular connective tissue capsule with inward septa that carry vessels and support the parenchyma.
2. Afferent lymphatic vessels
   Several valved vessels entering the convex surface and opening into the subcapsular sinus.
3. Subcapsular (marginal) sinus
   Space beneath the capsule receiving afferent lymph; lined by sinus macrophages that capture particulate antigen and pass it to B cells in follicles.
4. Cortex and lymphoid follicles
   Outer B-cell zone containing primary and secondary (germinal-centre) follicles organised by CXCL13 from follicular dendritic cells.
5. Paracortex
   Deep cortex: T-cell zone with dendritic cells presenting antigen, organised by CCL19/CCL21; contains HEVs. Thymus-dependent zone — depleted in DiGeorge syndrome.
6. High endothelial venules
   Post-capillary venules with plump cuboidal endothelium expressing peripheral-node addressin; entry point for naive T and B cells binding via L-selectin and CCR7.
7. Medullary cords
   Branching cords of plasma cells, macrophages and lymphocytes between the sinuses; plasma cells secrete antibody into efferent lymph.
8. Medullary sinuses
   Wide channels lined by endothelium and macrophages converging on the hilum.
9. Hilum and efferent vessel
   Indented region where the efferent lymphatic leaves and the nodal artery and vein enter/exit.

Anatomy

Arterial supply

One or two small arteries enter at the hilum, run in the medullary cords and trabeculae, and form capillary beds in the cortex and paracortex; these drain into high endothelial venules and then into veins that leave at the hilum.

Lymphatic drainage

Efferent vessels pass to the next node in the chain and eventually to a lymphatic trunk.

Histology

Capsule, trabeculae and reticular meshwork (silver stain shows reticular fibres); cortex with darker primary follicles and secondary follicles with pale germinal centres and a mantle zone; paracortex with HEVs (cuboidal endothelium, lymphocytes in transit within the wall); medulla with cords and sinuses. Sinuses contain sinus histiocytes (macrophages).

Development

Develop from the 2nd month as mesenchyme and lymphoid-tissue-inducer (LTi) cells, signalling through lymphotoxin, invade the primitive lymph sacs and lymphatic plexuses; lymphocytes colonise them later and germinal centres appear only after birth with antigen exposure.

Function

• Filters lymph, removing microbes, particles and cancer cells by macrophage phagocytosis.
• Concentrates antigen and antigen-presenting cells so rare antigen-specific lymphocytes can find them.
• Site of T-cell activation (paracortex) and B-cell activation, germinal-centre reactions and antibody production (cortex and medulla).
• Releases activated lymphocytes, memory cells and antibody into efferent lymph and hence the blood.
• Concentrates lymph by allowing some fluid to be reabsorbed into nodal blood vessels.

Physiology

Naive lymphocytes enter mostly via HEVs (L-selectin → CCR7 chemokine signalling → LFA-1/ICAM-1 arrest), sample antigen for hours to a day, then leave via efferent lymph guided by sphingosine-1-phosphate. B cells home to follicles (CXCR5–CXCL13) and T cells to the paracortex (CCR7–CCL19/21). Dendritic cells arrive in afferent lymph 1–2 days after antigen exposure. Activated T and B cells meet at the follicle–paracortex border, where T-cell help initiates germinal centres and extrafollicular plasma-cell responses; reactive nodes therefore enlarge within days.

Clinical significance

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