Chapter 20 – The Lymphatic System and Lymphoid Organs & Tissues

Overview of the Lymphatic System

• A meandering, one-way network of lymphatic vessels that returns interstitial fluid to the cardiovascular system.
• Fluid terminology
  • Interstitial fluid → enters lymphatic vessels → becomes lymph ("clear water").
• Structural–functional bridge to immunity
  • Lymphoid organs/tissues (spleen, thymus, tonsils, MALT, lymph nodes) provide the structural basis of the immune system.
  • Lymph nodes act as the keystone, linking vessel function with immune functions.

Components of the System

Lymphatic Vessels

• Functions
  • Return excess tissue fluid to blood.
  • Return leaked plasma proteins → maintain plasma colloid osmotic pressure.
  • Transport absorbed fats (via lacteals) from small intestine to blood.
  • Carry pathogens & antigens to lymph nodes.
• One-way flow: always toward the heart.

Lymphatic Capillaries

• Microscopic, blind-ended tubes interwoven with blood capillary beds.
• Absent in bone, bone marrow, teeth; present in limited areas of CNS meninges (drain brain interstitial & CSF).
• High permeability due to two specializations
  1. Overlapping endothelial cells form flap-like minivalves (one-way swing doors).
  2. Collagen anchoring filaments tether endothelial cells; ↑ interstitial volume → pulls flaps open.
• Admit large particles (proteins, cell debris, pathogens, cancer cells).
• Specialized intestinal capillaries = lacteals; lymph inside is chyle (milky, fat-laden).

Collecting Vessels → Trunks → Ducts

• Same 3 tunics as veins; thinner walls, more valves, frequent anastomoses.
• Superficial vessels travel with superficial veins; deep lymphatics track deep arteries.
• Major trunks: paired lumbar, bronchomediastinal, subclavian, jugular + single intestinal trunk.
• Terminal ducts
  • Right lymphatic duct → drains right upper limb, right head, right thorax; empties at junction of right internal jugular & right subclavian veins.
  • Thoracic duct (larger) → drains remainder; often begins as cisterna\,chyli (enlarged sac) at T{12}-L{2} level; empties into left venous junction.

Lymph Transport Mechanics

• No pump; low-pressure system.
• Propulsion mechanisms
  • Skeletal-muscle milking.
  • Respiratory thoracic pressure changes.
  • Pulsations of adjacent arteries.
  • Rhythmic contraction of smooth muscle in vessel walls (except smallest).
• Flow is sporadic & slow; physical movement ↑ rate.

Quantitative Highlights

• Fluid left behind at capillary beds ≈ 3\;\text{L day}^{-1} → must be reclaimed.

Homeostatic Imbalances (Vessels)

• Lymphangitis: inflamed lymphatics; vasa vasorum congest → red tender streaks.
• Lymphedema: blocked/removed lymphatics → severe localized edema.

Lymphoid Cells & Tissues

Cellular Players

• Lymphocytes
  • T cells: manage immune response; directly attack infected cells.
  • B cells: differentiate into plasma cells → secrete antibodies that tag antigens.
• Macrophages: phagocytize & activate T cells.
• Dendritic cells: capture antigens, migrate to nodes, present to T cells.
• Reticular cells: produce reticular fiber stroma scaffold.

Tissue Types

• Diffuse lymphoid tissue: loose arrangement; nearly every body organ (lamina propria of mucosae).
• Lymphoid follicles (nodules): solid, spherical B-cell bodies; possess germinal centers.
  • Isolated as Peyer’s patches & appendix follicles or as part of larger organs (nodes, tonsils).

Lymphoid Organs

• Primary: sites of lymphocyte maturation
  • Red bone marrow → origin & B-cell maturation.
  • Thymus → T-cell maturation.
• Secondary: first antigen encounter & activation
  • Lymph nodes, spleen, MALT (tonsils, Peyer’s patches, appendix).
  • Only lymph nodes filter lymph directly (afferent & efferent vessels).

Lymph Nodes

Distribution

• Hundreds; largest clusters in cervical, axillary, inguinal regions where vessels converge.

Functions

1. Filter lymph: macrophages remove microorganisms/debris before lymph returns to blood.
2. Immune activation: site where lymphocytes encounter antigens (often delivered by dendritic cells).

Histological Architecture

• Size: < 2.5\;\text{cm} (≈ 1\;\text{inch}), bean-shaped.
• Capsule → inward trabeculae create compartments.
• Two regions
  • Cortex: follicles with germinal centers (dividing B cells); deeper cortex rich in T cells; abundant dendritic cells.
  • Medulla: medullary cords (T & B cells) + medullary sinuses.
• Sinuses: subcapsular → cortical → medullary; criss-crossed by reticular fibers laden with macrophages.

Lymph Flow Pattern

\text{Afferent vessels} \rightarrow \text{subcapsular sinus} \rightarrow \text{cortical sinuses} \rightarrow \text{medullary sinuses} \rightarrow \text{hilum} \rightarrow \text{efferent vessels}

• Fewer efferent than afferent routes → stagnation → increased filtration & immune interaction time.

Clinical Notes

• Buboes: infected, pus-filled nodes (e.g., bubonic plague).
• Cancer metastasis often seeds nodes; nodes become hard, non-tender, enlarged.

Spleen

Position & Blood Supply

• Left upper abdomen, curling around stomach; supplied by splenic artery & vein (branches of celiac trunk & hepatic portal system respectively).

Functions

• Immune surveillance & lymphocyte proliferation.
• Blood cleanser
  • Removes aged/defective RBCs & platelets.
  • Macrophages phagocytize debris & pathogens.
• Additional roles
  • Salvages & stores iron; sends other Hb breakdown products to liver.
  • Stores \sim 1/3 of body’s platelets + monocytes.
  • Fetal erythrocyte production (may persist in adults under stress).

Microanatomy

• White pulp: lymphocyte-rich islands around central arteries → immune functions.
• Red pulp: splenic cords + sinusoids; site of RBC & pathogen destruction.

Pathology

• Thin capsule; rupture → intraperitoneal hemorrhage; historically splenectomy, now conservative management.
• Splenomegaly: enlargement (e.g., mononucleosis, malaria, leukemia, septicemia).

MALT (Mucosa-Associated Lymphoid Tissue)

• Scattered, strategically placed lymphoid tissue in mucous membranes → guards entry points.
• Major aggregations
  1. Tonsils (pharyngeal ring)
  • Palatine (largest, most often infected), lingual, pharyngeal (adenoids when enlarged), tubal.
  • Tonsillar crypts trap bacteria → immune memory formation.
  1. Peyer’s patches: large nodules in distal small intestine wall.
  2. Appendix: tubular offshoot of cecum; high follicle density.
• Additional diffuse MALT in respiratory, genitourinary, and rest of GI tracts.

Thymus

Location & Development

• Inferior neck → superior thorax, deep to sternum; bilobed.
• Prominent in newborns; enlarges during first year; starts involuting at puberty; residual tissue persists, still produces some T cells.

Structure

• Lobulated (like cauliflower).
  • Cortex: densely packed, rapidly dividing T lymphocytes + macrophages.
  • Medulla: fewer lymphocytes + thymic corpuscles (Hassall’s corpuscles) → regulatory T-cell development.
• Epithelial-cell stroma, not reticular fibers.
• Blood–thymus barrier isolates developing T cells from antigens.
• Lacks B cells & follicles; no direct antigen fight—serves strictly for T-cell maturation.

Comparative Summary (Key Differentiators)

• Capsule present: nodes, spleen, thymus; MALT generally lacks full capsule (tonsils partially encapsulated).
• Lymphoid follicles rich in B cells: nodes (cortex), spleen (white pulp), MALT; absent in thymus.
• Stroma: reticular connective tissue (nodes, spleen, MALT) vs epithelial tissue (thymus).
• Unique features: efferent+afferent vessels (nodes), red/white pulp (spleen), crypts (tonsils), cisterna chyli (duct), thymic corpuscles (thymus).

Numerical & Equation Snapshot

• Daily capillary fluid loss → 3\;\text{L} returns via lymph.
• Lymph node length < 2.5\;\text{cm} (≈ 1\;\text{in}).
• Platelet reservoir in spleen ≈ \frac{1}{3} total platelets.

Developmental Notes

• Lymphatic vessels & main node clusters arise by week 5 embryonically from lymph sacs budding off veins.
• Thymus (endodermal) first lymphoid organ; others (mesodermal mesenchyme) follow; lymphocytes populate organs shortly after birth.

Selected Clinical & Vocabulary Terms

• Elephantiasis: lymphedema due to filarial worms.
• Hodgkin’s lymphoma: malignant, Reed–Sternberg cells, high cure rate.
• Non-Hodgkin’s lymphoma: all other lymphoid cancers; variable prognosis.
• Sentinel node: 1st draining node examined for cancer spread.
• Tonsillitis: inflamed/infected tonsils.
• Lymphangiography: imaging with radiopaque dye.

Integrated Questions / Self-Check Prompts

1. Define lymph; trace its origin path.
2. Identify regions drained by right lymphatic vs thoracic ducts.
3. Summarize forces propelling lymph.
4. Analyze edema development when lymphatics are blocked—refer to capillary hydrostatic & colloid osmotic pressures.
5. Distinguish primary vs secondary lymphoid organs.
6. Sketch & label a lymph node (afferent, efferent, hilum, cortex, medulla, follicles).
7. Explain why fewer efferent vessels benefit immune surveillance.
8. Relate splenic artery/vein to celiac trunk & hepatic portal vein hierarchy.
9. Predict consequences of thymic removal in a neonate vs an adult.

These bullet-point notes encompass every major and minor concept in the transcript, elaborate on complex ideas, include quantitative data in LaTeX, integrate clinical correlations, and connect developmental and functional perspectives—forming a comprehensive stand-alone study guide for Chapter 20 on the lymphatic system.