Lymphatic System — Comprehensive Checklist

Hierarchical breakdown: System → Category → Subcategory → Individual Structure/Component

Organisational principle: Anatomical first, functional second. The lymphatic system lends itself to anatomical organisation — organs, vessels, and regional node groups are physically discrete structures. Functional detail (immune cell populations, specialised transport) is nested within the anatomical framework.

1. Thymus

Function: T-cell maturation and selection. Bone marrow-derived progenitors enter the thymus, undergo positive selection (can they bind MHC?) and negative selection (do they attack self?), and exit as mature, self-tolerant T-cells. ~95-98% of thymocytes die during selection (apoptosis).

1.1 Gross Structure

• Right lobe — larger in most individuals
• Left lobe
• Thymic capsule — dense connective tissue enclosing both lobes
• Interlobular septa — fibrous extensions dividing each lobe into lobules
• Corticomedullary junction — boundary zone where mature T-cells exit via venules; site of progenitor entry

1.2 Cortex (Outer)

• Thymocytes (immature T-cells) — dense population undergoing TCR gene rearrangement and positive selection
• Cortical epithelial cells (nurse cells) — present self-MHC to thymocytes; those that bind survive (positive selection)
• Macrophages — clear apoptotic thymocytes that fail selection (~95% of all thymocytes)

1.3 Medulla (Inner)

• Hassall’s corpuscles — concentric whorls of keratinised epithelial cells; produce TSLP, involved in Treg generation
• Medullary epithelial cells — express AIRE (AutoImmune REgulator) gene; present tissue-specific antigens for negative selection (delete self-reactive T-cells)
• Mature T-cells (single-positive) — CD4+ or CD8+, ready for peripheral export
• Dendritic cells — additional negative selection checkpoint; present peripheral self-antigens

2. Bone Marrow (Red/Yellow)

Function: Primary haematopoietic organ. All blood cells (including lymphocyte precursors) originate here. B-cells mature here (positive and negative selection). T-cell progenitors migrate to thymus for maturation. NK cells complete development here.

2.1 Red Marrow (Haematopoietically Active)

• Haematopoietic stem cells (HSCs) — self-renewing, multipotent; give rise to all blood lineages
• Common lymphoid progenitors (CLPs) — committed to lymphoid lineage (B, T, NK, ILC)
• B-cell precursors — sequential stages: pro-B (heavy chain rearrangement) → pre-B (surrogate light chain) → immature B (surface IgM); self-reactive cells deleted or edited
• NK cell precursors — mature in marrow; no antigen-specific receptor rearrangement
• Stromal cells — reticular cells (scaffold), adipocytes (niche regulation), endothelial cells (sinusoidal lining, HSC niche)
• Sinusoidal capillaries — fenestrated endothelium allowing mature cell egress into blood

2.2 Yellow Marrow (Adipose-Dominant)

• Fatty replacement of haematopoietic tissue — increases with age
• Retains capacity for reconversion to red marrow under haematopoietic stress (severe anaemia, chronic blood loss)

2.3 Key Anatomical Sites

• Sternum — common biopsy site (sternal puncture)
• Iliac crest — preferred biopsy site (trephine biopsy)
• Vertebral bodies — major haematopoietic reserve in adults
• Proximal femur/humerus — red marrow in adults, yellow distally
• Ribs — active haematopoiesis throughout life
• Skull (diploe) — spongy bone between inner and outer tables

3. Spleen

Function: Filters blood (not lymph). Immunological: captures blood-borne antigens, initiates immune responses to systemic infections, produces antibodies (especially against encapsulated bacteria). Haematological: removes senescent/damaged RBCs, recycles iron, reservoir for monocytes and platelets.

3.1 White Pulp (Immune Compartment)

• Periarteriolar lymphoid sheath (PALS) — T-cell zone surrounding central arterioles; T-cell activation by dendritic cells presenting blood-borne antigens
• Lymphoid follicles — B-cell zone
  • Primary follicles (naive B-cells) — resting, antigen-unexposed
  • Secondary follicles (germinal centres) — active immune response
    • Dark zone — centroblasts undergoing rapid proliferation and somatic hypermutation (diversifying antibody genes)
    • Light zone — centrocytes tested for improved antigen binding (affinity selection); losers apoptose
    • Follicular dendritic cells (FDCs) — trap and display antigen-antibody complexes on surface (not MHC); guide B-cell selection
    • Tingible body macrophages — phagocytose apoptotic B-cells that fail affinity selection
• Marginal zone — interface between white and red pulp; first contact with blood-borne antigens
  • Marginal zone B-cells — T-independent responses, rapid IgM production against polysaccharide antigens (encapsulated bacteria: pneumococcus, meningococcus, H. influenzae)
  • Marginal zone macrophages — capture particulate antigens from blood
  • Marginal metallophilic macrophages — line inner margin, capture viral particles and relay to dendritic cells

3.2 Red Pulp (Filtration Compartment)

• Splenic cords (cords of Billroth) — loose reticular tissue; macrophages test RBC deformability
• Splenic sinusoids — narrow-slit endothelium; old/rigid RBCs cannot squeeze through → phagocytosed
• Littoral cells — specialised sinusoidal endothelium; express both endothelial and macrophage markers
• Red pulp macrophages — erythrophagocytosis, iron recycling (haemoglobin → bilirubin + iron → transferrin)

3.3 Structural Framework

• Splenic capsule — dense connective tissue with myofibroblasts (contractile in some species)
• Trabeculae — internal fibrous extensions carrying trabecular vessels
• Splenic hilum — entry/exit point for splenic artery, vein, nerves, lymphatics

3.4 Vasculature

• Splenic artery → trabecular arteries → central arteries (within PALS) → penicillar arterioles → splenic sinusoids → trabecular veins → splenic vein
• Open circulation model — blood flows through cords before re-entering sinusoids (filtration mechanism)

4. Mucosal & Diffuse Lymphoid Tissue (MALT)

Function: Immune surveillance and response at mucosal surfaces — the largest interface between body and environment (~400m² combined surface area). Specialised for secretory IgA production (transcytosed across epithelium), antigen sampling from luminal contents via M-cells, and maintaining tolerance to commensals while responding to pathogens.

4.1 Gut-Associated Lymphoid Tissue (GALT)

Function: Immune monitoring of intestinal contents. Balances two opposing needs: tolerance to food antigens and commensal bacteria vs. rapid response to pathogens. The gut contains more lymphocytes than all other lymphoid organs combined.

4.1.1 Organised GALT (Structured Lymphoid Aggregates)

• Peyer’s patches (ileum — 100-200 patches)
  • Dome epithelium with M-cells (microfold cells) — transcytose luminal antigens across epithelium to underlying immune cells; no mucus layer, no glycocalyx, thin membrane
  • Subepithelial dome (SED) — dendritic cells and macrophages receive transcytosed antigens, process and present to T/B-cells
  • B-cell follicles with germinal centres — IgA class switching occurs here; activated B-cells become IgA-committed plasmablasts
  • Interfollicular T-cell zones — CD4+ T-cell activation; Tfh cells support B-cell class switching to IgA
• Isolated lymphoid follicles (ILFs) — scattered throughout small and large intestine; smaller than Peyer’s patches, inducible; contribute to IgA production
• Appendix — dense lymphoid aggregates; proposed function as reservoir of commensal-specific IgA-producing cells for recolonisation after gut flora disruption

4.1.2 Diffuse GALT (Non-Organised Immune Cells)

• Intraepithelial lymphocytes (IELs) — mostly CD8+ T-cells and γδ T-cells; reside between epithelial cells; rapid cytotoxic response to infected/stressed epithelial cells; ~1 IEL per 4-10 epithelial cells
• Lamina propria lymphocytes — mixed population (T-cells, B-cells, plasma cells, macrophages, dendritic cells, mast cells, eosinophils); lamina propria plasma cells produce ~3-5g of secretory IgA daily (most produced immunoglobulin in the body)
• Cryptopatches — small clusters of lineage-negative lymphoid precursors at crypt bases; precursors for ILFs and ILC3 cells

4.1.3 Associated Lymph Nodes

• Mesenteric lymph nodes — largest lymph node chain in the body; drain the entire intestine; site of oral tolerance induction (tolerogenic dendritic cells present food antigens to generate Tregs)

4.2 Bronchus-Associated Lymphoid Tissue (BALT)

Function: Respiratory tract immune surveillance. Not constitutively present in healthy human adults — induced by infection or inflammation (iBALT). When present, functions like Peyer’s patches for the airway.

• Bronchial lymphoid follicles — B-cell aggregates with germinal centres at bronchial bifurcations
• Inducible BALT (iBALT) — forms in response to pulmonary infection, autoimmunity, or chronic inflammation; organised with T/B zones, HEVs, FDCs
• Bronchial intraepithelial lymphocytes — analogous to gut IELs; surveillance of airway epithelium

4.3 Nasopharynx-Associated Lymphoid Tissue (NALT)

Function: First immunological contact point for inhaled and ingested antigens. Waldeyer’s ring guards the entrance to both respiratory and GI tracts.

4.3.1 Waldeyer’s Tonsillar Ring

• Palatine tonsils (paired) — largest; lateral oropharynx
  • Tonsillar crypts — deep invaginations increasing surface area 300-fold; trap antigens; lined with specialised M-cell-like epithelium
  • Germinal centres — B-cell activation, IgA/IgG class switching
  • Stratified squamous epithelium — non-keratinised; infiltrated with lymphocytes
• Pharyngeal tonsil (adenoid) — single, posterior nasopharynx roof; pseudostratified ciliated epithelium; prominent in children, involutes after puberty
• Lingual tonsils — base of tongue; multiple small nodules; persist throughout life
• Tubal tonsils (paired) — near Eustachian tube openings; protect middle ear from ascending nasopharyngeal infections

4.3.2 Nasal Lymphoid Tissue

• Nasal-associated lymphoid tissue — diffuse lymphoid aggregates in nasal mucosa; antigen sampling of inspired air

4.4 Skin-Associated Lymphoid Tissue (SALT)

Function: Immune surveillance of the skin barrier (~1.8m² surface area). Detects pathogens penetrating the epidermis and initiates immune responses. Key in contact hypersensitivity and wound defence.

• Langerhans cells (epidermis) — resident dendritic cells; capture antigens, migrate via afferent lymphatics to draining nodes; present on MHC-II to T-cells
• Dermal dendritic cells — deeper population; broader antigen capture; produce cytokines driving Th1/Th17 responses
• Skin-homing T-cells (CLA+ T-cells) — cutaneous lymphocyte antigen marks T-cells that preferentially home to skin; resident memory T-cells (TRM) provide rapid local recall
• Draining cutaneous lymph nodes — receive antigens from skin via dermal lymphatics

4.5 Other MALT

Function: Immune protection at specialised mucosal surfaces not covered by the major MALT subtypes above.

• Conjunctiva-associated lymphoid tissue (CALT) — protects ocular surface; follicles in conjunctival fornix; produces secretory IgA in tear film
• Lacrimal drainage-associated lymphoid tissue (LDALT) — lymphoid aggregates in lacrimal sac and nasolacrimal duct; guards drainage pathway to nasal cavity
• Salivary gland-associated lymphoid tissue — IgA-producing plasma cells in parotid, submandibular, sublingual glands; salivary IgA is first antimicrobial defence in oral cavity
• Vulvovaginal-associated lymphoid tissue — mucosal immune defence of lower female reproductive tract; IgA and IgG secretion; altered by hormonal cycling
• Urogenital-associated lymphoid tissue — lymphoid aggregates in ureters and bladder mucosa; defence against ascending urinary tract infections

5. Lymphatic Vasculature & Circulation

Function: A one-way drainage system returning interstitial fluid, immune cells, and macromolecules to the venous circulation. Maintains fluid homeostasis (prevents oedema), transports dietary lipids (lacteals → chyle), and serves as the highway for immune cell trafficking and antigen delivery to lymph nodes.

5.1 Initial Lymphatics (Lymphatic Capillaries)

Function: Entry point for interstitial fluid. Uniquely permeable — designed to absorb fluid, proteins, and cells that blood capillaries cannot reabsorb. The one-way valve design prevents backflow into the interstitium.

• Blind-ended endothelial tubes — closed distal ends create absorption pockets in tissue
• Anchoring filaments (to surrounding ECM) — when tissue swells (oedema), filaments pull endothelial flaps open, increasing fluid uptake
• Overlapping endothelial flaps (primary valves) — function as one-way microvalves; interstitial pressure pushes flaps open, intraluminal pressure pushes them closed
• Discontinuous basement membrane — gaps allow large molecules (proteins, lipids) and cells (dendritic cells, lymphocytes) to enter
• Oak-leaf shaped endothelial cells — distinctive morphology; overlapping edges create the flap valves
• Button-like junctions — intermittent junctions between endothelial cells (vs. continuous “zipper” junctions in blood capillaries); the gaps between buttons permit fluid and cell entry

5.2 Pre-collecting Vessels (Pre-collectors)

Function: Transitional segment. Begin to develop contractile capacity and valves, starting to actively propel lymph rather than passively absorb it.

• Transition from capillaries to collectors — mixed features (some button junctions, some zipper junctions)
• Scattered smooth muscle cells — begin intrinsic contractile pumping
• Intraluminal valves (begin here) — first appearance of true bicuspid valves preventing retrograde flow

5.3 Collecting Lymphatic Vessels

Function: Active lymph transport. Smooth muscle contracts rhythmically (intrinsic pump), augmented by skeletal muscle compression, arterial pulsation, and respiratory movements (extrinsic pump). Lymphangions are the functional contractile units.

• Wall layers:
  • Intima (endothelium + basement membrane) — continuous basement membrane (unlike capillaries); produces nitric oxide regulating smooth muscle tone
  • Media (smooth muscle — circular and longitudinal) — spontaneous pacemaker-driven contractions; frequency increases with stretch (filling)
  • Adventitia (connective tissue) — anchors vessel, carries vasa vasorum and nervi vasorum
• Lymphangions (functional contractile units between valves) — each segment between two valves acts as a mini-pump; contracts 6-10 times/minute
• Bicuspid intraluminal valves — ensure unidirectional flow toward ducts; failure → lymphoedema
• Vasa vasorum — blood supply to vessel wall (in larger collectors)
• Autonomic nerve supply (sympathetic, parasympathetic, sensory) — modulates contractile frequency and tone; noradrenaline increases contraction, NO decreases it

5.4 Lymphatic Trunks (Major Collecting Trunks)

Function: Final convergence of regional drainage before emptying into the two main ducts. Each trunk drains a major body region.

• Jugular trunks (paired) — drain head and neck; receive efferent lymph from deep cervical nodes
• Subclavian trunks (paired) — drain upper limbs; receive from apical axillary nodes
• Bronchomediastinal trunks (paired) — drain thoracic viscera, chest wall, lungs; receive from mediastinal and tracheobronchial nodes
• Lumbar trunks (paired) — drain lower limbs, pelvis, abdominal wall, kidneys, adrenals, gonads; receive from lumbar/para-aortic nodes
• Intestinal trunk(s) — drain GI tract, liver, spleen, pancreas; carry chyle (lipid-rich lymph); unpaired, variable anatomy
• Descending intercostal trunks — drain lower intercostal spaces; variable drainage to thoracic duct or lumbar trunks

5.5 Lymphatic Ducts

Function: Final conduits returning all lymph to the venous system. The thoracic duct handles ~75% of body lymph; the right lymphatic duct handles ~25%.

5.5.1 Thoracic Duct (Ductus Thoracicus)

• Cisterna chyli (L1-L2 level, when present) — dilated sac receiving lumbar trunks and intestinal trunk; reservoir for chyle
• Abdominal segment (through aortic hiatus) — ascends posterior to aorta through diaphragm
• Thoracic segment (posterior mediastinum) — ascends between aorta and azygos vein; crosses from right to left at ~T5
• Cervical segment (arch at C7-T1) — arches laterally behind carotid sheath
• Termination: left venous angle (junction of left internal jugular + left subclavian veins) — re-enters venous circulation
• Drains: left side of head/neck, left upper limb, left thorax, both lower limbs, abdomen, pelvis (~75% of body)

5.5.2 Right Lymphatic Duct

• Short (~1.25 cm) or absent (trunks drain separately into veins) — most variable lymphatic structure
• Termination: right venous angle
• Drains: right side of head/neck, right upper limb, right hemithorax (~25% of body)

5.6 Lymph (Fluid)

Function: The transport medium of the lymphatic system. Carries immune cells, antigens, proteins, and waste products. Composition varies by origin: peripheral lymph is protein-poor; intestinal lymph (chyle) is lipid-rich.

5.6.1 Composition

• Interstitial fluid (filtered plasma) — ~2-4 litres/day returned to circulation
• Lymphocytes (predominantly T-cells) — recirculating between blood, nodes, and tissues
• Macrophages and dendritic cells — antigen-loaded, migrating to lymph nodes
• Proteins (albumin, globulins, fibrinogen — lower concentration than plasma) — molecules too large for venous capillary reabsorption
• Lipids (chylomicrons — in lacteals) — dietary long-chain fatty acids packaged by enterocytes
• Electrolytes — similar to plasma composition
• Waste products and cellular debris — metabolic waste, apoptotic cell fragments
• Antigens and immune complexes — foreign material draining from infection/inflammation sites

5.6.2 Special Forms

• Chyle — milky, lipid-rich lymph from intestinal lacteals; triglyceride content 1-5 g/dL; gives thoracic duct lymph its characteristic milky appearance post-meal
• Aqueous humour drainage (Schlemm’s canal — lymphatic-like) — maintains intraocular pressure; blockage → glaucoma
• Cerebrospinal fluid (meningeal lymphatics / glymphatic drainage) — recently discovered (2015) CNS waste clearance pathway; active during sleep; drains to deep cervical nodes

5.7 Lymphovenous Return

Function: The critical junction where lymph re-enters the blood. Failure at these junctions (thrombosis, surgical damage, compression) causes lymphoedema.

• Junction of thoracic duct with left venous angle — primary return point; valve prevents venous blood reflux into duct
• Junction of right lymphatic duct with right venous angle — secondary return point
• Lymphovenous communications (peripheral — pathological or developmental) — in lymphoedema, new lymphovenous shunts may form as compensatory drainage; surgically created lymphovenous anastomoses (LVA) used to treat lymphoedema

6. Regional Lymph Nodes

Function: Maps the drainage territories of the body. Clinically critical for: understanding infection spread (lymphadenitis follows drainage), cancer staging (metastasis follows lymphatic drainage — sentinel node biopsy), and surgical planning (lymph node dissection boundaries).

6.1 Lymph Node Structure (Microanatomy)

Function: Filter lymph. Trap antigens draining from peripheral tissues, present them to lymphocytes, initiate adaptive immune responses. Sentinel function: first nodes in a drainage basin encounter antigens first (sentinel node concept).

6.1.1 Cortex (Outer — B-cell Zone)

• Subcapsular sinus — first receiving chamber for afferent lymph; subcapsular sinus macrophages capture particulate antigens and relay to B-cell follicles
• Cortical sinuses — channels allowing lymph percolation through cortex
• B-cell follicles — same primary/secondary follicle structure as spleen (see 3.1)
  • Primary follicles — naive B-cells awaiting antigen
  • Secondary follicles (germinal centres) — activated B-cells undergoing somatic hypermutation, class switching, affinity maturation
• Follicular dendritic cells — antigen display for B-cell selection (same role as in spleen)

6.1.2 Paracortex (Deep Cortex — T-cell Zone)

• T-cell zone — densest T-cell population in the node; site of T-cell activation by dendritic cells
• High endothelial venules (HEVs) — specialised postcapillary venules; naive lymphocytes extravasate from blood into node here (L-selectin, CCR7-mediated homing)
• Interdigitating dendritic cells — arrived from peripheral tissues via afferent lymphatics; present processed antigens on MHC to naive T-cells

6.1.3 Medulla (Effector Zone)

• Medullary cords — plasma cells (actively secreting antibodies into efferent lymph), macrophages (final filtration checkpoint)
• Medullary sinuses — converging channels draining toward hilum; carry antibodies and effector cells to efferent lymphatic

6.1.4 Structural & Circulatory Framework

• Capsule — collagenous outer layer
• Afferent lymphatics — multiple, entering at convex surface; deliver antigens and dendritic cells
• Efferent lymphatics — single, exiting at hilum; carry antibodies, effector lymphocytes, filtered lymph
• Hilum — entry point for arterial blood supply and nerve fibres; exit point for venous drainage and efferent lymphatic
• Reticular network (fibroblastic reticular cells — FRCs) — 3D scaffold creating corridors (conduits) that guide lymphocyte migration and channel small soluble antigens directly to dendritic cells

6.2 Head & Neck

Function: Drain scalp, face, oral cavity, pharynx, larynx, thyroid, salivary glands. The deep cervical chain is the final common pathway — virtually all head/neck lymph passes through it before reaching the jugular trunk.

• Occipital nodes — drain posterior scalp
• Retroauricular (mastoid) nodes — drain posterior ear, temporal scalp
• Preauricular (parotid) nodes — drain lateral face, eyelids, temporal region, external ear
• Submandibular nodes — drain oral cavity floor, anterior tongue, lower lip, cheeks, nose, submandibular gland
• Submental nodes — drain chin, lower lip centre, floor of mouth tip, anterior tongue tip
• Superficial cervical nodes
  • Anterior cervical chain — drain anterior neck, superficial structures
  • Posterior cervical chain — drain posterior neck, occipital scalp overflow
• Deep cervical nodes
  • Superior deep cervical (jugulodigastric / tonsillar node) — drain tonsils, tongue base; first node to enlarge in tonsillar infection
  • Inferior deep cervical (jugulo-omohyoid / Virchow’s node — left) — left supraclavicular; enlargement suggests abdominal/thoracic malignancy (Troisier’s sign)
• Supraclavicular nodes — drain thorax, abdomen (via thoracic duct on left); palpable supraclavicular node is always pathological until proven otherwise
• Retropharyngeal nodes — drain nasopharynx, middle ear, paranasal sinuses; not palpable clinically
• Pretracheal and paratracheal nodes — drain thyroid, larynx, trachea

6.3 Upper Limb

Function: Drain hand, forearm, arm, breast, anterior chest wall. Axillary nodes are the critical staging group for breast cancer (Level I-III dissection).

• Epitrochlear (cubital) nodes — drain medial hand and forearm; palpable in sarcoidosis, lymphoma, secondary syphilis
• Axillary nodes:
  • Pectoral (anterior) group — Level I; drain breast, anterior chest wall
  • Subscapular (posterior) group — Level I; drain posterior shoulder, scapular region
  • Lateral (humeral) group — Level I; drain upper limb
  • Central group — Level II; receive from all Level I groups
  • Apical (infraclavicular/subclavicular) group — Level III; final axillary station before subclavian trunk
• Deltopectoral (infraclavicular) nodes — drain lateral arm, deltoid region; relay to apical axillary

6.4 Thorax

Function: Drain lungs, mediastinal structures, chest wall, diaphragm, breast (deep drainage). Tracheobronchial nodes are critical in lung cancer staging and sarcoidosis.

• Parasternal (internal mammary) nodes — drain medial breast, anterior chest wall, upper abdominal wall; involved in breast cancer spread to contralateral side
• Intercostal nodes — drain posterior chest wall; relay to thoracic duct or posterior mediastinal nodes
• Superior diaphragmatic nodes
  • Prepericardial — drain anterior diaphragm, liver surface
  • Lateral pericardial — drain lateral diaphragm
• Anterior mediastinal nodes — drain thymus, pericardium, anterior mediastinal structures
• Posterior mediastinal nodes — drain oesophagus, posterior pericardium, thoracic vertebral bodies
• Tracheobronchial nodes
  • Superior tracheobronchial — drain upper lobes
  • Inferior tracheobronchial (carinal/subcarinal) — drain lower lobes; key in lung cancer staging
  • Bronchopulmonary (hilar) nodes — first nodes draining lung parenchyma; coal dust deposition → anthracosis
  • Intrapulmonary nodes — within lung parenchyma along bronchi

6.5 Abdomen & Pelvis

Function: Drain GI tract, liver, spleen, pancreas, kidneys, adrenals, reproductive organs, bladder, pelvic floor. Para-aortic nodes are critical in staging of testicular, ovarian, and uterine cancers.

• Coeliac nodes — drain stomach, liver, spleen, proximal duodenum, pancreas
• Superior mesenteric nodes — drain jejunum, ileum, caecum, ascending and transverse colon
• Inferior mesenteric nodes — drain descending colon, sigmoid, upper rectum
• Lumbar (para-aortic / lateral aortic) nodes — drain kidneys, adrenals, gonads (testicular/ovarian drainage follows gonadal vessels to para-aortic nodes, NOT inguinal)
  • Pre-aortic — drain GI viscera (receive from coeliac, SMA, IMA nodes)
  • Lateral aortic — drain kidneys, adrenals, gonads, body wall
  • Retroaortic — drain posterior body wall
• Common iliac nodes — drain external and internal iliac territories, lower rectum
• External iliac nodes — drain lower limb (via inguinal), bladder dome, uterine body, prostate
• Internal iliac (hypogastric) nodes — drain pelvic viscera: rectum, prostate, cervix, vagina, bladder base
• Obturator nodes — within obturator fossa; drain prostate, bladder neck; critical in prostate cancer staging
• Sacral nodes — drain rectum, posterior pelvic wall
• Inguinal nodes:
  • Superficial inguinal (horizontal and vertical groups) — horizontal drain lower abdominal wall, perineum, external genitalia, gluteal region; vertical drain superficial lower limb
  • Deep inguinal (node of Cloquet/Rosenmuller) — most superior deep inguinal node; gateway to external iliac chain

6.6 Lower Limb

Function: Drain foot, leg, thigh. Follow the superficial (great/small saphenous) and deep (femoral, popliteal) venous systems. Lymphoedema of the lower limb is the most common form globally (filariasis in tropics, post-surgical in developed world).

• Popliteal nodes (superficial and deep) — drain posterolateral leg, knee joint; superficial follow small saphenous vein, deep follow popliteal vessels
• Anterior tibial node (inconstant) — drain anterior leg; along anterior tibial vessels
• Inguinal nodes (see 6.5) — final station for lower limb drainage

7. Immune Cell Populations

Function: The cellular components that populate and operate within lymphatic tissue. Lymphocytes (adaptive immunity) and myeloid cells (innate immunity and antigen presentation) work together to mount immune responses.

7.1 Lymphocytes

7.1.1 T-cells (Thymus-Derived)

• CD4+ Helper T-cells — orchestrate immune responses by cytokine secretion; “conductors” of the immune system
  • Th1 — activate macrophages, drive cell-mediated immunity (intracellular pathogens); produce IFN-γ, TNF-α
  • Th2 — drive antibody production (especially IgE), eosinophil activation (parasites, allergy); produce IL-4, IL-5, IL-13
  • Th17 — neutrophil recruitment, mucosal defence (fungal, extracellular bacteria); produce IL-17, IL-22
  • Tfh (follicular helper) — essential for germinal centre reactions; help B-cells undergo class switching and affinity maturation; produce IL-21
  • Treg (regulatory) — suppress excessive immune responses, maintain self-tolerance, prevent autoimmunity; produce IL-10, TGF-β; express FoxP3
• CD8+ Cytotoxic T-cells — kill virus-infected cells and tumour cells directly; recognise peptide-MHC-I complexes; release perforin (pore-forming) and granzymes (trigger apoptosis)
• Gamma-delta (γδ) T-cells — bridge innate and adaptive immunity; recognise stress ligands and phosphoantigens without MHC restriction; abundant in epithelial tissues (skin, gut, lung)
• NKT cells (Natural Killer T-cells) — recognise lipid antigens presented by CD1d (not MHC); rapid cytokine burst (IFN-γ, IL-4) within hours of activation; regulatory role in autoimmunity and tumour surveillance
• MAIT cells (Mucosal-Associated Invariant T-cells) — recognise vitamin B metabolites from bacteria/yeast via MR1; abundant in liver, blood, mucosal tissues; rapid antimicrobial response

7.1.2 B-cells (Bone Marrow-Derived)

• Naive B-cells — antigen-unexposed; circulate between blood and lymphoid follicles; express surface IgM and IgD
• Memory B-cells — long-lived (decades); rapid recall response on re-exposure; can express any isotype; basis of vaccine-mediated protection
• Plasma cells (antibody-secreting) — terminally differentiated; secrete ~2,000 antibodies/second; migrate to bone marrow (long-lived) or mucosal tissues (IgA-producing); no surface immunoglobulin
• Plasmablasts — short-lived, recently activated antibody-secreting cells; circulate in blood during acute immune responses; precursors to long-lived plasma cells
• B-1 cells (innate-like) — produce natural antibodies (polyreactive IgM) without prior antigen exposure; reside in peritoneal and pleural cavities; first-line defence against common pathogens
• Marginal zone B-cells — reside in splenic marginal zone; rapid T-independent response to blood-borne polysaccharide antigens (encapsulated bacteria); produce IgM
• Regulatory B-cells (Breg) — suppress inflammatory responses via IL-10, IL-35, TGF-β; modulate autoimmunity and transplant rejection

7.1.3 NK Cells (Natural Killer Cells)

• CD56bright (cytokine-producing) — ~10% of blood NK; dominant in lymph nodes and tissues; produce IFN-γ, TNF-α; immunoregulatory role; low cytotoxicity
• CD56dim (cytotoxic) — ~90% of blood NK; express CD16 (FcγRIII); kill via antibody-dependent cellular cytotoxicity (ADCC) and natural cytotoxicity (missing-self recognition: kill cells that downregulate MHC-I, e.g., virus-infected or tumour cells); release perforin/granzymes

7.1.4 Innate Lymphoid Cells (ILCs)

• ILC1 — produce IFN-γ; mirror Th1 function; defence against intracellular pathogens; tissue-resident
• ILC2 — produce IL-5, IL-13; mirror Th2 function; parasite defence, allergy, tissue repair; respond to epithelial alarmins (IL-25, IL-33, TSLP)
• ILC3 — produce IL-17, IL-22; mirror Th17 function; mucosal defence (gut), lymphoid tissue organogenesis; include lymphoid tissue inducer (LTi) cells

7.2 Myeloid Cells in Lymphatic Tissue

Function: Antigen presentation, phagocytosis, and innate immune signalling within lymphoid organs. Bridge innate and adaptive immunity.

• Macrophages (tissue-resident, subcapsular sinus) — phagocytose pathogens and debris; present antigens on MHC-II; produce cytokines (IL-1, IL-6, TNF-α) activating inflammation; subcapsular sinus macrophages in lymph nodes are the first to capture incoming antigens from afferent lymph
• Dendritic cells — professional antigen-presenting cells; most potent activators of naive T-cells
  • Conventional DC1 (cDC1) — cross-present exogenous antigens on MHC-I to CD8+ T-cells; critical for anti-tumour and antiviral immunity
  • Conventional DC2 (cDC2) — present on MHC-II to CD4+ T-cells; drive Th1, Th2, Th17 differentiation
  • Plasmacytoid DC (pDC) — produce massive amounts of type I interferon (IFN-α/β) in response to viral nucleic acids; antiviral sentinel
• Follicular dendritic cells (FDCs) — NOT haematopoietic (stromal origin); do NOT process antigens; trap intact antigen-antibody complexes on their surface via Fc and complement receptors; display these to B-cells in germinal centres for affinity selection; retain antigen for months-years (immune memory maintenance)
• Monocytes — circulating precursors that enter tissues and differentiate
  • Patrolling (non-classical, CD14lowCD16+) — survey endothelium; early detection of vascular damage/infection
  • Inflammatory (classical, CD14+CD16−) — recruited to sites of infection/inflammation; differentiate into macrophages or dendritic cells
• Mast cells — tissue-resident; granules contain histamine, heparin, tryptase; IgE receptor (FcεRI) crosslinking triggers degranulation; allergic responses, parasite defence; resident in lymph node T-cell zones, modulate T-cell activation

8. Specialised Lymphatic Structures

Function: Structures with unique lymphatic functions that don’t fit neatly into the organ, vessel, or node categories above.

8.1 Lacteals

Function: Specialised lymphatic capillaries in intestinal villi dedicated to dietary lipid absorption. Long-chain fatty acids (>12 carbons) and fat-soluble vitamins (A, D, E, K) are packaged as chylomicrons by enterocytes and are too large for blood capillary absorption — they enter lacteals instead.

• Central lacteal (villus of small intestine) — single blind-ended lymphatic in each villus; villus smooth muscle contraction (muscularis mucosae) rhythmically squeezes lacteal to propel chyle
• Absorb dietary long-chain fatty acids and fat-soluble vitamins (A, D, E, K) — medium-chain fatty acids bypass lacteals and enter portal blood directly
• Transport chylomicrons → mesenteric lymphatics → cisterna chyli → thoracic duct → left venous angle → systemic circulation — dietary fat enters blood as chylomicrons without first-pass hepatic metabolism

8.2 Meningeal/Glymphatic System

Function: Waste clearance from the CNS. The brain has no conventional lymphatic vessels within its parenchyma. Instead, CSF-mediated flow through perivascular spaces (glymphatic system) clears metabolic waste (including amyloid-β), draining to meningeal lymphatics along dural sinuses and ultimately to cervical lymph nodes. Dysfunction implicated in Alzheimer’s disease, aging, and post-traumatic neuroinflammation. Most active during sleep.

• Meningeal lymphatic vessels (dural sinuses) — discovered 2015; line dural sinuses (superior sagittal, transverse); drain CSF, immune cells, and waste to deep cervical lymph nodes
• Glymphatic pathway (perivascular/paravascular spaces) — CSF flows along arterial perivascular spaces into brain parenchyma, exchanges with interstitial fluid (via AQP4 water channels on astrocyte endfeet), exits along venous perivascular spaces; bulk flow clears solutes
• Cervical lymph node drainage of CNS antigens — deep cervical nodes are the immune surveillance point for CNS; CNS antigens reaching these nodes can trigger neuroinflammatory responses
• Cribriform plate drainage (nasal lymphatics) — CSF drains through the cribriform plate along olfactory nerve sheaths to nasal mucosa lymphatics; significant proportion of total CSF drainage

8.3 Lymphatic Valves

Function: Ensure unidirectional lymph flow from periphery to venous system. Without functioning valves, lymph would pool in dependent tissues under gravity. Valve incompetence is a primary mechanism in lymphoedema.

• Intraluminal bicuspid valves — two leaflets of endothelial cells with connective tissue core; open in direction of flow, close under retrograde pressure
• Endothelial cell-derived — valve leaflets continuous with vessel endothelium; formed during lymphatic development under PROX1/FOXC2 transcription factor control; FOXC2 mutations → lymphoedema-distichiasis syndrome
• Prevent retrograde flow — critical in upright posture; lower limb lymph must travel against gravity ~1 metre to reach inguinal nodes
• Density increases distally — highest concentration in lower limbs (reflecting greater hydrostatic challenge); progressively fewer valves in proximal trunks and ducts