Lymphatic System

Overview

The lymphatic system operates in parallel with and complementary to the cardiovascular system. Unlike the closed cardiovascular circuit, the lymphatics form a one-way drainage network. The critical driving pressure is tissue interstitial pressure: Starling forces at capillary beds net ~3 L/day of protein-rich plasma filtrate into the interstitium that venous capillary reabsorption cannot fully reclaim — the lymphatics collect and return this fluid, preventing progressive oedema.

The system's immune function is equally fundamental: lymphatic capillaries are the highway by which antigen-bearing dendritic cells (DCs) travel from peripheral tissues to regional lymph nodes — the anatomical encounter point where naïve T and B cells are activated to initiate adaptive immune responses. Dietary fat absorption adds a third dimension: chylomicrons (75–1200 nm) are too large for blood capillary junctions and exclusively enter intestinal lacteals → thoracic duct → systemic circulation, bypassing first-pass hepatic metabolism.

Key Components

Key Functions

Fluid homeostasis (Starling forces): At the arterial end of capillaries, hydrostatic pressure (~35 mmHg) exceeds oncotic pressure (~28 mmHg) → net filtration (~20 mL/min systemic). At the venous end, net reabsorption is slightly less → ~3 L/day accumulates in interstitium → must be drained by lymphatics to prevent progressive oedema. Failure → lymphoedema.

Immune surveillance: The lymph node is the critical encounter point between antigen-bearing DCs (arriving via afferent lymph, guided by CCL19/21 gradient through CCR7 expression) and recirculating naïve lymphocytes (entering via HEV using L-selectin/PNAd tethering → LFA-1/ICAM-1 arrest). DC-T cell cognate interaction (TCR-pMHC + CD28-B7 + cytokines) → T cell activation → clonal expansion → Tfh-B cell interaction → germinal centre → affinity maturation → class switch → plasma cells and memory B cells exit via efferent lymphatics → blood → effector tissues.

Dietary lipid transport: Enterocytes package dietary TG + cholesterol + apoB-48 into chylomicrons (75–1,200 nm) — too large for blood capillary junctions. Chylomicrons enter intestinal lacteals via button junctions → mesenteric lymphatics → cisterna chyli → thoracic duct → left subclavian vein. This explains why fat-soluble vitamins (A, D, E, K) and lipophilic drugs initially bypass hepatic first-pass metabolism.

Lymph Node Immune Architecture

  Tissue injury / pathogen
         │ DC maturation → CCR7 ↑
         ▼
  Afferent lymphatic → subcapsular sinus
  (SCS macrophages trap large antigens + cell debris;
   relay to follicular DCs and B cells)
         │
         ▼
  Paracortex (T-cell zone)
  DC presents antigen on MHC-I (CD8 CTL) / MHC-II (CD4 Th)
  + CD28-B7 co-stimulation + cytokine signals
         │ T cell activation + clonal expansion
         │ Tfh cells differentiate → migrate to follicle
         ▼
  Secondary follicle → GERMINAL CENTRE
  Centroblasts (rapid division + somatic hypermutation)
    → centrocytes compete for FDC antigen
    → affinity selection → class switch recombination
         │
         ├─► Long-lived plasma cells (bone marrow)
         │   → high-affinity IgG/IgA/IgE
         └─► Memory B cells (circulate indefinitely)
         │
  Medullary cords: plasma cell IgG/IgA secretion
  Efferent lymphatic: antibodies + effector cells → blood

Spleen — Haematological and Immune Functions

Pathology

Cross-Atlas Connections

References

• Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 14th ed. Elsevier; 2021. elsevier.com
• Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 7th ed. W.W. Norton; 2022. NCBI Bookshelf
• Swartz MA. The physiology of the lymphatic system. Adv Drug Deliv Rev. 2001;50(1-2):3-20. doi:10.1016/S0169-409X(01)00150-8
• WHO. Lymphatic Filariasis. who.int
• Morton LM, et al. Lymphoma Incidence Patterns by WHO Subtype in the United States. Blood. 2006;107(1):265-276. doi:10.1182/blood-2005-06-2508