Today let's do inflammation and tissue repair don't have time for explanation though because it's too long can give an overview
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I now have enough content from Robbins to build a solid overview. Let me compile it.
Inflammation & Tissue Repair - Overview
Source: Robbins, Cotran & Kumar - Pathologic Basis of Disease
π₯ ACUTE INFLAMMATION
Definition: A rapid-onset host response to injury, infection, or dead cells designed to eliminate the offending agent.
3 Core Components:
- Vasodilation β increased blood flow (heat + redness)
- Increased vascular permeability β plasma proteins leak out (swelling)
- Leukocyte emigration β neutrophils recruited to destroy the cause
Cardinal Signs: Rubor (redness), Calor (heat), Tumor (swelling), Dolor (pain), Functio laesa (loss of function)
Vascular Changes
| Step | What Happens |
|---|---|
| Vasodilation | Histamine acts on smooth muscle of postcapillary venules β increased flow |
| Increased permeability | Plasma-rich exudate leaks into tissue; high protein = exudate (vs. transudate = low protein, osmotic/hydrostatic cause) |
| Stasis | Slower blood flow β red cells pack β leukocytes marginate to endothelium |
Leukocyte Recruitment (Step by Step)
- Rolling - selectins (P-selectin, E-selectin on endothelium) bind sialyl-Lewis X on leukocytes
- Activation - chemokines activate leukocytes, upregulate integrin affinity
- Firm adhesion - LFA-1/MAC-1 (integrins) bind ICAM-1/VCAM-1 on endothelium
- Transmigration - leukocytes squeeze through endothelium via CD31 (PECAM-1)
- Chemotaxis - leukocytes migrate along gradient of C5a, LTB4, IL-8, bacterial products
Time course: Neutrophils dominate 0-24 hours β monocytes/macrophages take over at 24-48 hours (neutrophils are short lived, monocytes survive longer, and some stimuli preferentially attract monocytes)
Phagocytosis & Killing
- Recognition/opsonization - IgG, C3b coat microbes for better phagocyte binding
- Engulfment - pseudopods engulf β phagosome β phagolysosome
- Killing - two arms:
- O2-dependent: NADPH oxidase β superoxide β HβOβ β HOCl (myeloperoxidase). Called the "respiratory burst"
- O2-independent: lysozyme, defensins, elastase, lactoferrin
Neutrophil Extracellular Traps (NETs): chromatin + antimicrobial proteins released extracellularly to trap bacteria (also cause collateral tissue damage)
Mediators of Inflammation
| Mediator | Source | Key Action |
|---|---|---|
| Histamine | Mast cells, basophils, platelets | Vasodilation + increased permeability (early) |
| Serotonin | Platelets | Similar to histamine |
| Prostaglandins (PGEβ, PGIβ) | Arachidonic acid via COX | Vasodilation, fever, pain |
| Leukotrienes (LTBβ) | AA via lipoxygenase | Chemotaxis; LTCβ/Dβ/Eβ β bronchoconstriction |
| TNF + IL-1 | Macrophages, DCs | Endothelial activation, adhesion molecules, fever, acute phase response |
| IL-6 | Macrophages | Acute phase proteins (via liver) |
| Chemokines (IL-8/CXCL8) | Macrophages, endothelium | Leukocyte chemotaxis + adhesion |
| C3a, C5a | Complement | Mast cell activation, chemotaxis, opsonization (C3b) |
| PAF | Leukocytes, endothelium | Platelet aggregation, vasoconstriction |
| Nitric oxide (NO) | Endothelium, macrophages | Vasodilation, microbicidal |
Arachidonic Acid Pathway (high-yield):
- COX pathway β Prostaglandins + Thromboxane Aβ
- Lipoxygenase pathway β Leukotrienes (LTBβ = chemotaxis; LTCβ/Dβ/Eβ = bronchoconstriction/allergy)
- NSAIDs block COX; corticosteroids block both by inhibiting phospholipase Aβ
Morphologic Patterns
| Pattern | Features | Example |
|---|---|---|
| Serous | Thin, protein-poor fluid | Blister, pleural effusion |
| Fibrinous | Fibrin-rich exudate | Fibrinous pericarditis ("bread and butter") |
| Purulent/Suppurative | Pus (neutrophils + debris) | Abscess |
| Ulcer | Surface epithelium excavated | Peptic ulcer |
Outcomes of Acute Inflammation
- Complete resolution - offending agent eliminated, tissue returns to normal (limited injury + regenerating tissue)
- Healing by scarring (fibrosis) - extensive destruction or non-regenerative tissue β connective tissue replaces parenchyma
- Progression to chronic inflammation - agent persists or healing is impaired
π°οΈ CHRONIC INFLAMMATION
Defined by: lymphocytes + macrophages (not neutrophils), tissue destruction, and attempts at repair going on simultaneously
Causes:
- Persistent infections (TB, fungi, certain parasites)
- Autoimmune diseases
- Prolonged toxic exposure (silica, foreign bodies)
Key Cells:
- Macrophages - dominant cell; classically activated (M1) by IFN-Ξ³ β kill microbes; alternatively activated (M2) by IL-4/IL-13 β tissue repair
- Lymphocytes - T and B cells; bidirectional cross-talk with macrophages perpetuates the reaction
- Th1 (IFN-Ξ³) β macrophage classical activation
- Th2 (IL-4, IL-5, IL-13) β eosinophil recruitment, IgE, alternative macrophage activation
- Th17 (IL-17) β neutrophil + monocyte recruitment
- Plasma cells - produce antibodies
- Eosinophils - parasite infections, allergic inflammation
- Mast cells - allergic + innate responses
Granulomatous Inflammation
A specific pattern of chronic inflammation characterized by clusters of activated macrophages (epithelioid cells) + multinucleated giant cells + rim of lymphocytes
When? When an agent resists killing - TB (caseous necrosis in center), fungal infections, foreign bodies, sarcoidosis (non-caseating), Crohn's disease
Systemic Effects of Inflammation (Acute Phase Response)
Driven mainly by IL-1, TNF, IL-6 acting on the hypothalamus and liver:
- Fever - cytokines β PGEβ in hypothalamus β raise temperature set point
- Leukocytosis - colony-stimulating factors release marrow reserves
- Bacterial infection β neutrophilia; viral β lymphocytosis; parasites/allergy β eosinophilia
- Acute phase proteins - liver produces CRP, fibrinogen, serum amyloid A (SAA)
- CRP opsonizes microbes; fibrinogen causes ESR elevation
- Septic shock - massive TNF + IL-1 β DIC, hypotension, metabolic derangements
π©Ή TISSUE REPAIR
Repair = restoration of architecture and function after injury. Two mechanisms:
1. Regeneration (Restitution)
Cell types by proliferative potential:
| Type | Examples | Capacity |
|---|---|---|
| Labile | Epithelia (skin, GI, respiratory), hematopoietic | Continuously dividing; contain stem cells |
| Stable | Hepatocytes, renal tubular cells, fibroblasts | Normally quiescent; can proliferate on demand |
| Permanent | Neurons, cardiac myocytes, skeletal muscle | Cannot divide; rely on stem cells or hypertrophy |
Liver regeneration is the classic example: cytokines + growth factors (HGF, EGF) stimulate surviving hepatocytes to divide; progenitor cells also contribute when damage is massive.
2. Repair by Scarring
When regeneration fails (permanent tissue, damaged ECM scaffold, or extensive necrosis), connective tissue fills in.
Steps in Scar Formation:
- Hemostasis (minutes) - platelet plug β stable clot = scaffold
- Inflammation (6-48 h) - neutrophils then monocytes clear debris
- Cell proliferation (up to 10 days):
- Epithelial cells migrate to cover wound
- Fibroblasts proliferate, deposit collagen
- Endothelial cells form new vessels β angiogenesis
- Granulation tissue - fibroblasts + new capillaries + loose ECM (pink, soft, granular; VEGF drives the angiogenesis)
- Remodeling (weeks-months) - collagen cross-linked, Type III β Type I collagen, scar contracts via myofibroblasts
Key growth factors in repair:
- VEGF - angiogenesis
- PDGF, TGF-Ξ² - fibroblast migration + collagen synthesis
- EGF, HGF - epithelial regeneration
- TGF-Ξ² - also suppresses inflammation and drives fibrosis
Wound Healing
| Feature | Primary Intention | Secondary Intention |
|---|---|---|
| Wound edges | Approximated (clean surgical cut) | Wide gap, tissue loss |
| Granulation tissue | Minimal | Abundant |
| Scarring | Minimal | Large scar |
| Contraction | Minor | Major (myofibroblasts) |
Factors That Impair Healing
| Factor | Mechanism |
|---|---|
| Diabetes | Poor circulation, impaired neutrophil function, glycosylation of proteins |
| Malnutrition (esp. Vit C deficiency) | Inadequate collagen synthesis (scurvy) |
| Corticosteroids | Suppress inflammation + fibroblast activity |
| Infection | Persistent inflammation, tissue destruction |
| Poor blood supply | Hypoxia limits fibroblasts and angiogenesis |
| Mechanical stress | Dehiscence |
Abnormalities in Tissue Repair
- Deficient scarring: Dehiscence (wound rupture), ulceration, chronic wounds (especially in diabetics/ischemia)
- Excessive scarring:
- Hypertrophic scar - raised but stays within wound boundary; can regress
- Keloid - grows beyond wound margins, does not regress; more common in darkly pigmented skin
- Fibrosis/Cirrhosis - excessive collagen in parenchymal organs (liver cirrhosis, pulmonary fibrosis, renal fibrosis)
Quick memory anchor:
Acute = fast, neutrophils, vascular β Chronic = slow, macrophages/lymphocytes, granulomas β Repair = regenerate if you can, scar if you can't
- Robbins, Cotran & Kumar - Pathologic Basis of Disease, Ch. 3
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