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Inflammation — A Complete Guide

Based on Robbins, Cotran & Kumar: Pathologic Basis of Disease

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🔷 What Is Inflammation?

Think of it like the body's 911 system — it detects a problem, calls in reinforcements, eliminates the threat, and then tries to repair the damage.

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🔷 Causes of Inflammation

• PAMPs — Pathogen-Associated Molecular Patterns (bacterial lipopolysaccharide, viral dsRNA)
• DAMPs — Damage-Associated Molecular Patterns (ATP, uric acid, HMGB1 released from dying cells)

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PART 1 — ACUTE INFLAMMATION

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🔸 Definition

1. Vascular dilation → increased blood flow
2. Increased vascular permeability → plasma proteins exit into tissues
3. Leukocyte (mainly neutrophil) emigration → accumulate and destroy the offending agent

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🔸 The Five Cardinal Signs (Celsus + Virchow)

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🔸 Step 1 — Vascular Reactions

A. Changes in Flow & Caliber

1. Transient vasoconstriction — lasts only seconds
2. Vasodilation (mainly postcapillary venules) — driven by histamine → redness + heat
3. Increased permeability — plasma proteins pour into tissues → exudate forms → swelling
4. Stasis — blood slows as fluid is lost; red cells concentrate → vessel engorgement
5. Leukocytes line up (margination) along activated endothelium

B. Exudate vs. Transudate

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🔸 Step 2 — Leukocyte Recruitment

🅐 Margination & Rolling

• With stasis, neutrophils drift to the vessel periphery (margination)
• They loosely roll along activated endothelium via selectins (P-selectin, E-selectin on endothelium binding sialyl-Lewis X on leukocytes)

🅑 Adhesion (Firm Arrest)

• Chemokines activate leukocytes → upregulate integrins (LFA-1, MAC-1)
• Integrins bind ICAM-1 on endothelium → firm adhesion

🅒 Transmigration (Diapedesis)

• Leukocytes squeeze between endothelial cells (through tight junctions) by binding PECAM-1 (CD31)
• They then cross the basement membrane using collagenases

🅓 Chemotaxis

• Leukocytes migrate along a chemical gradient toward the injury site
• Key chemoattractants: C5a (complement), LTB₄ (leukotriene B4), IL-8/CXCL8 (chemokine), bacterial products (fMLP)

Simple memory trick: Roll → Stick → Squeeze through → Crawl toward the problem (RSSC)

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🔸 Step 3 — Phagocytosis

• Phagocytes use opsonins to boost binding — IgG (via Fc receptor), C3b (via complement receptor), mannose-binding lectin
• Opsonization = coating a target to make it easier to eat

• Cytoplasmic extensions (pseudopods) surround the particle → form a phagosome → fuses with lysosome → phagolysosome

• Reactive Oxygen Species (ROS): NADPH oxidase generates superoxide (O₂⁻) → H₂O₂ → hypochlorous acid (HOCl, the most potent) in the "respiratory burst"
• Reactive Nitrogen Species (RNS): iNOS generates nitric oxide (NO) → reacts with O₂⁻ → peroxynitrite (ONOO⁻)
• Lysosomal enzymes: Proteases, elastase, myeloperoxidase, defensins

Chediak-Higashi syndrome = defective lysosome-phagosome fusion → recurrent infections Chronic Granulomatous Disease (CGD) = NADPH oxidase defect → no respiratory burst → catalase-positive organisms survive

Neutrophil Extracellular Traps (NETs)

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🔸 Mediators of Acute Inflammation

Cell-Derived Mediators

Plasma-Derived Mediators

NSAIDs (aspirin, ibuprofen) inhibit COX enzymes → block prostaglandin synthesis → reduce fever, pain, inflammation Corticosteroids inhibit phospholipase A₂ → block both prostaglandin AND leukotriene synthesis

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🔸 Morphologic Patterns of Acute Inflammation

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🔸 Outcomes of Acute Inflammation

Acute Inflammation
       │
  ┌────┴──────────────────┐
  ▼                       ▼
Complete Resolution    Cannot Resolve
(short injury, minimal  ────────────────────────
tissue destruction)         │              │
Macrophages clear debris    ▼              ▼
Lymphatics resorb fluid  Scarring     Chronic
Tissue regenerates      (fibrosis)   Inflammation

1. Complete resolution — The ideal outcome. Debris cleared, edema resorbed, tissue regenerated back to normal.
2. Healing by connective tissue (scarring/fibrosis) — When tissue can't regenerate (e.g., heart muscle, neurons) or when fibrin can't be cleared.
3. Progression to chronic inflammation — When the injurious agent persists or healing is impaired.

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PART 2 — CHRONIC INFLAMMATION

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🔸 Definition

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🔸 Causes of Chronic Inflammation

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🔸 Morphologic Features of Chronic Inflammation

1. Mononuclear cell infiltrate — macrophages, lymphocytes, and plasma cells (NOT neutrophils)
2. Tissue destruction — caused by the persistent offending agent AND by the inflammatory cells themselves
3. Repair — angiogenesis + fibrosis trying to fill the damage

The battlefield never clears — fighting and rebuilding happen at the same time.

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🔸 Key Cells in Chronic Inflammation

1. Macrophages — The Central Generals

• Circulating monocytes (from bone marrow)
• Tissue-resident macrophages seeded from yolk sac/fetal liver before birth (Kupffer cells in liver, microglia in brain, alveolar macrophages in lung)

• M1 (Classical activation) — by IFN-γ and microbial products → produce ROS, NO, TNF, IL-1, IL-12 → kill microbes → pro-inflammatory
• M2 (Alternative activation) — by IL-4, IL-13 (from Th2 cells) → secrete IL-10, TGF-β → tissue repair, fibrosis, anti-inflammatory

• Secreting cytokines (TNF, IL-1, IL-12) that perpetuate the response
• Presenting antigens to T lymphocytes
• Producing growth factors that stimulate fibrosis (TGF-β, PDGF)

2. Lymphocytes — The Memory Soldiers

• CD4+ T helper cells are the main lymphocytes in chronic inflammation
  • Th1 cells produce IFN-γ → activate macrophages (classical M1 activation)
  • Th2 cells produce IL-4, IL-5, IL-13 → activate eosinophils + alternative macrophage activation
  • Th17 cells produce IL-17 → recruit neutrophils
• CD8+ cytotoxic T cells kill infected cells directly
• Macrophage–T cell crosstalk forms a self-amplifying loop in diseases like TB and sarcoidosis

3. Other Cells

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🔸 Granulomatous Inflammation — A Special Pattern

• TB — caseating granuloma (central cheesy necrosis) + Langhans giant cells
• Sarcoidosis — non-caseating granuloma (no necrosis)
• Crohn's disease — non-caseating granuloma
• Leprosy, Cat-scratch disease, Schistosomiasis, Fungal infections

Granuloma = body's way of "walling off" what it cannot destroy

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🔸 Systemic Effects of Inflammation ("Acute Phase Response")

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🔷 Acute vs. Chronic Inflammation — Quick Comparison

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🔷 Key Takeaway (Robbins Summary)

Inflammation is a double-edged sword — it is essential for host defense and healing, but when it goes unchecked, it becomes the engine of widespread tissue destruction. Virtually all chronic diseases — atherosclerosis, type 2 diabetes, cancer, Alzheimer's, autoimmune disorders — have inflammation as a central driver.

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