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PA3.1-2 | Acute Inflammation — Vascular & Cellular Events, Mediators — SDL Guide (Part 2)

Leucocyte Recruitment Cascade

Leucocyte emigration from blood to tissue is a multistep adhesion cascade. It is predominantly neutrophils in the first 6–24 hours; monocytes predominate after 24–48 hours.

Step 1 — Margination: As stasis develops, neutrophils move from the axial stream to the vessel periphery.

Step 2 — Rolling: Selectins mediate loose, reversible tethering:
• P-selectin: stored in Weibel–Palade bodies of endothelium; rapidly mobilised by histamine.
• E-selectin: induced on endothelium by IL-1 and TNF (hours later).
• L-selectin: constitutively expressed on leucocytes.
Selectins bind glycoprotein ligands (e.g., P-selectin glycoprotein ligand-1, PSGL-1).

Step 3 — Activation: Chemokines (e.g., IL-8/CXCL8) displayed on endothelial surface bind G-protein-coupled receptors on rolling neutrophils → signal transduction → integrin activation.

Step 4 — Firm adhesion: Activated integrins (LFA-1 / Mac-1; CD11a–CD18 / CD11b–CD18) bind ICAM-1 (intercellular adhesion molecule-1) and VCAM-1 on endothelium. This is high-affinity, stable arrest.

Step 5 — Transmigration (diapedesis): Neutrophils squeeze between endothelial cells (paracellular) at the junction, guided by PECAM-1 (CD31). They then cross the basement membrane using matrix metalloproteinases (collagenase).

Step 6 — Chemotaxis: Directional migration toward the injury site along a chemical gradient. Major chemotactic agents:
• C5a (complement)
• LTB₄ (leukotriene B4)
• IL-8 / CXCL8 (chemokine)
• Bacterial products (e.g., N-formyl-methionyl peptides, f-Met-Leu-Phe)

A multi-panel diagram shows neutrophils moving from blood to inflamed tissue through margination, rolling, activation, firm adhesion, diapedesis, and chemotaxis. — **Panel A:** Post-capillary venule, blood flow, red blood cells, axial stream, neutrophil margination, rolling, activation, firm adhesion, transmigration, chemotaxis, inflamed tissue, 6-24 h neutrophil predominance, 24-48 h monocyte predominance. **Panel B:** P-selectin, E-selectin, L-selectin, PSGL-1, endothelial cell, rolling neutrophil, histamine, IL-1, TNF. **Panel C:** IL-8/CXCL8, endothelial chemokine display, neutrophil GPCR, activated integrins, LFA-1, Mac-1, ICAM-1, VCAM-1, firm adhesion. **Panel D:** Endothelial junction, PECAM-1/CD31, basement membrane, matrix metalloproteinases, collagenase, chemotactic gradient, C5a, LTB4, IL-8/CXCL8, bacterial products, injury site.

CLINICAL PEARL

Leucocyte adhesion deficiency (LAD) is a rare autosomal recessive disorder caused by mutations in the CD18 gene, resulting in absent or dysfunctional β₂-integrins (LFA-1, Mac-1). Affected children suffer recurrent bacterial infections with markedly elevated blood neutrophil counts (neutrophilia) but NO pus formation at infection sites — because neutrophils cannot undergo firm adhesion or diapedesis. This directly illustrates why integrin–ICAM-1 binding is essential for effective leucocyte emigration.

Phagocytosis and Microbicidal Mechanisms

Diagram showing neutrophil phagocytosis from opsonin-mediated recognition through engulfment to oxygen-dependent and oxygen-independent killing inside the phagolysosome. — **Panel A:** Neutrophil, bacterium, opsonins, Fc-gamma receptor, CR1, pseudopods, phagosome, lysosome, phagolysosome. **Panel B:** IgG Fc fragment, C3b, mannose-binding lectin/collectins, Fc-gamma receptor, CR1 complement receptor, collectin receptor, opsonised bacterium. **Panel C:** NADPH oxidase, O2, superoxide O2 radical anion, SOD, H2O2, MPO, chloride ion, HOCl, hydroxyl radical, singlet oxygen, nitric oxide, BPI, lysozyme, lactoferrin, defensins, cathepsins, elastase, major basic protein, chronic granulomatous disease callout.

Once at the site, neutrophils (and macrophages) destroy pathogens via phagocytosis.

Phase 1 — Recognition and opsonisation:
Opsonins coat pathogens and enhance phagocyte binding:
• IgG Fc fragment — binds Fc-γR on phagocytes.
• C3b (complement) — binds CR1 (complement receptor 1).
• Collectins (mannose-binding lectin).

Phase 2 — Engulfment: Pseudopods extend and fuse around the particle → phagosome. The phagosome fuses with lysosomes → phagolysosome.

Phase 3 — Killing:

Oxygen-dependent (respiratory burst / oxidative burst):
• NADPH oxidase complex on the phagosome membrane: O₂ → superoxide (O₂•⁻) via electron transfer.
• Superoxide → H₂O₂ (hydrogen peroxide) spontaneously or via SOD.
• Myeloperoxidase (MPO) catalyses: H₂O₂ + Cl⁻ → hypochlorous acid (HOCl) — the most potent bactericidal agent in neutrophils.
• Also: •OH (hydroxyl radical), singlet O₂, NO.

Oxygen-independent:
• Bactericidal/permeability-increasing protein (BPI): disrupts gram-negative outer membrane.
• Lysozyme: cleaves peptidoglycan of gram-positive bacteria.
• Lactoferrin: chelates iron (bacteria need iron).
• Major basic protein: kills parasites (eosinophils).
• Defensins, cathepsins, elastase (from azurophil granules).

> Chronic granulomatous disease (CGD) — NADPH oxidase deficiency → defective respiratory burst → recurrent catalase-positive organism infections (Staph, Aspergillus).

SELF-CHECK

A child presents with recurrent deep-seated bacterial infections. His blood count shows marked neutrophilia, but wound biopsies show NO neutrophil infiltrate. Which molecule is most likely defective?

A. P-selectin on endothelial cells

B. CD18 (β₂-integrin subunit)

C. Myeloperoxidase in neutrophil granules

D. C3b complement fragment

Reveal Answer

Answer: B. CD18 (β₂-integrin subunit)

The clinical picture — high circulating neutrophils with absent tissue infiltration — is classic for Leucocyte Adhesion Deficiency (LAD), caused by CD18 (β₂-integrin) deficiency. Without functional LFA-1/Mac-1, neutrophils cannot achieve firm adhesion to ICAM-1 on endothelium and cannot transmigrate. P-selectin deficiency would impair rolling (less dramatic). MPO deficiency impairs killing, not migration. C3b deficiency affects opsonisation and chemotaxis.

Chemical Mediators — Overview

Mediators are the molecular language of acute inflammation. They are derived from plasma (inactive precursors → activated) or produced by cells (pre-formed or newly synthesised). Key principle: most mediators have short half-lives, are quickly inactivated, or have built-in counter-regulatory mechanisms — preventing excessive injury.

Cell-derived mediators:
• Vasoactive amines (pre-formed, immediate release)
• Arachidonic acid metabolites (newly synthesised, minutes)
• Platelet-activating factor (cell membrane–derived)
• Cytokines (synthesised de novo, hours)
• Reactive oxygen species and nitric oxide

Plasma-derived mediators (activated in plasma):
• Complement fragments
• Kinin system
• Coagulation/fibrinolytic system

A four-panel infographic classifies chemical mediators of acute inflammation by source, timing, actions, and clinical relevance around an inflamed venule. — **Panel A:** Inflamed post-capillary venule, endothelial cells, mast cell, macrophage, platelet, neutrophil, tissue injury, vasodilation, increased permeability, chemotaxis, fever, pain.. **Panel B:** Cell-derived mediators: vasoactive amines, arachidonic acid metabolites, platelet-activating factor, cytokines, reactive oxygen species, nitric oxide; plasma-derived mediators: complement fragments, kinin system, coagulation/fibrinolytic system.. **Panel C:** Immediate pre-formed mediator release, minutes for newly synthesised lipid mediators, hours for cytokines, short half-life, rapid inactivation, counter-regulation.. **Panel D:** Summary table of mediator group, source, main action, and clinical/pharmacological relevance including histamine, prostaglandins, leukotrienes, complement C3a/C5a, bradykinin, and TNF/IL-1..