Inflammation and Repair

• Definition: Inflammation is the response of vascularized tissue to infection or injury, delivering leukocytes and plasma proteins to the site of insult; repair is the subsequent restoration of tissue architecture by regeneration (parenchymal replacement) or scar formation (connective tissue deposition), or a mixture of both.
• Why the distinction matters: The outcome is dictated by the tissue's proliferative capacity and by whether the extracellular matrix scaffold survived the insult.
• Labile tissues (gut and skin epithelium, bone marrow): continuously cycling stem cells → true regeneration.
• Stable tissues (hepatocytes, renal tubules, fibroblasts): quiescent G0 cells that re-enter the cycle → regeneration if the reticulin framework is intact.
• Permanent tissues (cardiac myocytes, neurons, skeletal muscle): no meaningful replicative capacity → scar is the only outcome, which is why myocardial infarction yields a non-contractile fibrous scar rather than new myocardium.
• Clinical importance: Acute inflammation is protective but produces the morbidity of abscess, sepsis and ARDS when dysregulated; chronic inflammation is the substrate of atherosclerosis, rheumatoid arthritis, inflammatory bowel disease, chronic hepatitis, idiopathic pulmonary fibrosis, and inflammation-associated carcinogenesis (Barrett esophagus, chronic ulcerative colitis, chronic osteomyelitis).
• Epidemiology worth recalling: Chronic non-healing wounds are concentrated in older adults with diabetes, peripheral arterial disease, or immobility, and diabetic foot ulceration is a leading cause of non-traumatic lower-extremity amputation in the United States (IDSA diabetic foot infection guidance). Sarcoidosis, the prototypical non-caseating granulomatous disease, is substantially more common in Black Americans, especially women, and typically presents in the third to fifth decades. Tuberculosis, the prototypical caseating granulomatous disease, in the U.S. is concentrated among persons born outside the country and those with HIV or TNF-inhibitor exposure. Keloids cluster in individuals with darkly pigmented skin and show strong familial predisposition.

Causes grouped by mechanism

• Infection (PAMP-driven): Bacterial LPS, peptidoglycan, viral RNA and fungal glucans ligate TLRs and NOD-like receptors on macrophages → NF-κB activation → IL-1, IL-6, TNF-α. Pyogenic organisms (Staphylococcus aureus, Streptococcus pyogenes) drive neutrophil-rich suppurative inflammation; intracellular organisms (M. tuberculosis, Histoplasma, Bartonella) drive Th1/granulomatous inflammation.
• Tissue necrosis (DAMP-driven): Ischemia, trauma, burns and chemical injury release uric acid, ATP, HMGB1 and mitochondrial DNA → NLRP3 inflammasome → caspase-1 → mature IL-1β. This is sterile inflammation (post-MI pericarditis, gout).
• Foreign bodies: Suture, talc, silica, beryllium, keratin from a ruptured cyst → indigestible material → foreign-body granuloma with multinucleate giant cells.
• Immune-mediated: Autoimmunity (rheumatoid arthritis, Crohn disease), hypersensitivity (type IV in contact dermatitis and sarcoidosis-like reactions), and allergy sustain inflammation because the antigen cannot be cleared.

Non-modifiable risk factors examiners plant

• Age: Impaired angiogenesis, reduced fibroblast proliferation and immunosenescence → delayed healing and blunted febrile response.
• Inherited immune defects: Chronic granulomatous disease (NADPH oxidase), leukocyte adhesion deficiency type 1 (CD18/β2-integrin, with delayed umbilical cord separation and absent pus), Chédiak-Higashi (LYST, defective phagolysosome fusion).
• Connective tissue disease: Ehlers-Danlos (collagen defects), osteogenesis imperfecta.
• Skin phototype/family history: Keloid predisposition.

Modifiable risk factors

• Diabetes mellitus: Hyperglycemia impairs neutrophil chemotaxis and oxidative burst, glycates matrix, and adds microvascular ischemia and neuropathy.
• Glucocorticoids: Inhibit phospholipase A2, suppress fibroblast collagen synthesis → wound dehiscence.
• Smoking: Vasoconstriction, carboxyhemoglobin, reduced tissue oxygen tension needed for prolyl hydroxylase.
• Malnutrition: Vitamin C deficiency (prolyl/lysyl hydroxylation), zinc deficiency (matrix metalloproteinase cofactor), copper deficiency (lysyl oxidase cross-linking), protein deficiency.
• Ischemia, venous stasis, pressure, infection, radiation, and retained foreign body: The classic reasons a wound will not close.

Vascular phase — why the tissue looks red, hot and swollen

• Transient vasoconstriction, then arteriolar vasodilation: Histamine from mast cells, prostaglandins (PGD2, PGE2, PGI2) and nitric oxide relax precapillary sphincters → increased blood flow → rubor and calor.
• Increased vascular permeability: Histamine, bradykinin, C3a/C5a and leukotrienes C4/D4/E4 cause endothelial cell contraction in postcapillary venules → protein-rich exudate escapes → rising interstitial oncotic pressure → tumor (edema). Fibrinogen leakage yields the fibrinous exudate of pericarditis and the fibrin scaffold on which fibroblasts later migrate.
• Stasis and margination: Loss of plasma raises hematocrit and viscosity, slowing flow so leukocytes leave the axial stream.

Cellular phase — the adhesion cascade in order

• Rolling (selectins: E- and P-selectin on endothelium induced by TNF/IL-1, binding sialyl-Lewis X) → tight adhesion (β2-integrins LFA-1/Mac-1 binding ICAM-1, upregulated by chemokines) → transmigration (PECAM-1/CD31) → chemotaxis toward IL-8, C5a, LTB4 and bacterial N-formyl peptides.
• Killing: Opsonization by IgG and C3b → phagocytosis → NADPH oxidase respiratory burst → superoxide → myeloperoxidase converts H2O2 and Cl− to hypochlorite. Collateral release of neutrophil enzymes and reactive oxygen species produces the pus and tissue liquefaction of an abscess and drives pain (dolor) via bradykinin and PGE2 sensitization of nociceptors, with resulting functio laesa.

Transition to chronic inflammation and repair

• Persistent antigen → Th1 cells release IFN-γ → macrophages become epithelioid cells and fuse into Langhans giant cells, walled off by a lymphocyte cuff — a granuloma. TNF-α maintains the structure, which is why TNF blockade causes reactivation.
• Macrophages switch from M1 to M2, releasing TGF-β, PDGF and VEGF → angiogenesis plus fibroblast recruitment → granulation tissue (type III collagen) → remodeling by zinc-dependent matrix metalloproteinases into cross-linked type I collagen; myofibroblasts contract the scar.

Acute inflammation — the classic five signs, each with its mediator

• Rubor and calor: Histamine/prostaglandin/NO-mediated arteriolar vasodilation increases warm blood flow through the injured area.
• Tumor: Exudation of protein-rich fluid through leaky postcapillary venules; unlike a transudate, it is turbid with high protein and high LDH.
• Dolor: Bradykinin and PGE2 sensitize nociceptors; pressure from the exudate adds to it.
• Functio laesa: Pain plus edema restrict movement — the source of "guarding" in the abdominal stem.
• Systemic: Fever, malaise, anorexia and tachycardia from IL-1/IL-6/TNF; leukocytosis with a left shift and toxic granulation. A fluctuant, exquisitely tender, warm mass with overlying erythema is a drainable abscess.

Chronic inflammation — the indolent picture

• Low-grade fever, night sweats, weight loss, fatigue and normocytic anemia of chronic disease (IL-6 → hepcidin → iron trapped in macrophages, low serum iron, low TIBC, normal-to-high ferritin).
• Organ-specific fibrotic endpoints: exertional dyspnea with bibasilar Velcro crackles and clubbing in pulmonary fibrosis; ascites and portal hypertension in cirrhosis; strictures and fistulae in Crohn disease.

Exposures and demographics the stem names

• Recent immigrant, incarcerated person, HIV, or patient started on infliximab with cough, hemoptysis and apical cavitation → caseating granulomas of tuberculosis.
• Young Black woman with bilateral hilar lymphadenopathy, erythema nodosum, uveitis and hypercalcemia → sarcoidosis; the triad with fever and arthritis is Löfgren syndrome.
• Aerospace or ceramics worker → berylliosis, radiographically indistinguishable from sarcoidosis.
• Diabetic with a painless plantar ulcer over the metatarsal head → neuropathic wound failing to progress past the inflammatory phase.
• **Boy with recurrent abscesses from Staph aureus, Serratia, Burkholderia, Nocardia and *Aspergillus*** → chronic granulomatous disease.
• Raised, itchy scar extending beyond the original earlobe piercing or sternotomy incision in a patient with darkly pigmented skin → keloid, as opposed to a hypertrophic scar confined to the wound margins.

Step 1 — establish that inflammation is present and gauge acuity

• CBC with differential: Neutrophilia with left shift and band forms favors acute bacterial inflammation; lymphocytosis favors viral; eosinophilia suggests parasitic or allergic; monocytosis suggests chronic granulomatous processes.
• Acute phase reactants: CRP rises and falls within about a day of the stimulus and is the better marker of real-time activity; ESR rises and falls slowly (fibrinogen-driven rouleaux) and is used for indolent disease such as osteomyelitis, temporal arteritis and polymyalgia rheumatica. Neither is specific — they cannot distinguish infection from autoimmunity.
• Procalcitonin favors bacterial over viral infection but should support, not replace, clinical judgment.

Step 2 — localize and sample

• Imaging: Ultrasound distinguishes cellulitis from a drainable fluid collection; CT/MRI defines abscess, fistula and osteomyelitis. HRCT is the key study for interstitial fibrosis.
• Fluid analysis: An exudate has high protein and LDH — Light criteria in pleural fluid; a joint aspirate with markedly elevated WBC and neutrophil predominance suggests septic arthritis, and crystals under polarized light identify gout or pseudogout.
• Gram stain and culture before antibiotics whenever feasible, plus blood cultures if systemically ill.

Step 3 — histology, the gold standard for granulomatous and fibrotic disease

• Biopsy showing epithelioid macrophages with a lymphocyte rim defines a granuloma. Central caseous necrosis with positive acid-fast stain and culture (or NAAT) indicates tuberculosis; non-caseating granulomas with negative AFB and GMS/PAS stains suggest sarcoidosis, Crohn disease, berylliosis or foreign body.
• Per the ATS/ERS/WASOG sarcoidosis guideline, diagnosis requires compatible clinical and radiographic features plus non-caseating granulomas plus exclusion of alternatives; serum ACE is not recommended as a diagnostic test, and Löfgren syndrome may be diagnosed without biopsy. Scadding stages describe the chest radiograph.
• Per the ATS/ERS/JRS/ALAT IPF guideline, a UIP pattern on HRCT — subpleural, basal-predominant reticulation with honeycombing and traction bronchiectasis — in the right clinical setting can establish the diagnosis without surgical lung biopsy, ideally through multidisciplinary discussion.
• Pressure injuries are graded by the NPIAP staging system; latent TB must be excluded with a TST or IGRA before TNF inhibitors.

Immediate stabilization

• Sepsis or hemodynamic instability: Obtain cultures, give broad-spectrum antibiotics early and resuscitate with balanced crystalloid, then vasopressors (norepinephrine first-line) per the Surviving Sepsis Campaign.
• Suspected necrotizing soft tissue infection (pain out of proportion, crepitus, bullae, rapid progression): emergent surgical exploration and debridement — antibiotics alone are inadequate (IDSA skin and soft tissue infection guideline).

Source control first, drugs second

• Incision and drainage is the definitive treatment of an abscess; IDSA guidance adds systemic antibiotics for large or multiple lesions, surrounding cellulitis, systemic signs, or immunocompromise, with coverage of MRSA where prevalent. Retained foreign bodies, necrotic tissue and undrained collections must be removed or the inflammation will not resolve.
• Antimicrobials by pathogen: For tuberculous granulomas, CDC/ATS/IDSA-endorsed multidrug therapy (rifampin, isoniazid, pyrazinamide, ethambutol in the intensive phase); note that patients frequently worsen radiographically before improving.

Anti-inflammatory pharmacotherapy

• NSAIDs (e.g., ibuprofen): inhibit COX → less PGE2 → analgesia and antipyresis. Avoid in volume depletion or advanced CKD (afferent arteriolar constriction) and in active peptic ulcer disease.
• Glucocorticoids (e.g., prednisone): inhibit phospholipase A2 and NF-κB, suppressing both eicosanoid arms. First-line for symptomatic pulmonary or organ-threatening sarcoidosis per the ATS sarcoidosis treatment guideline. Contraindicated as monotherapy over an undrained abscess or untreated infection.
• Steroid-sparing and biologic escalation: methotrexate or azathioprine, then TNF-α inhibitors (infliximab) for refractory granulomatous disease. Screen for latent TB and hepatitis B before starting, per CDC recommendations, because TNF maintains granuloma integrity.

Repair and fibrosis

• Wound care: moist dressings, debridement, offloading and revascularization; per IDSA diabetic foot guidance, treat infection and correct ischemia rather than culturing uninfected ulcers.
• Antifibrotics: pirfenidone or nintedanib are conditionally recommended for IPF by the ATS/ERS/JRS/ALAT guideline; they slow FVC decline but do not reverse fibrosis. Lung transplantation is definitive.
• Keloids: intralesional corticosteroid injection first; excision alone risks recurrence.
• Nutritional correction of vitamin C, zinc and protein deficiency; smoking cessation.

Complications of unresolved acute inflammation

• Abscess and empyema: Walled-off neutrophilic liquefactive necrosis; antibiotics penetrate poorly, so persistent fever on appropriate therapy signals the need for drainage.
• Sepsis and septic shock — emergency: Systemic cytokine release with vasodilation, capillary leak and DIC; hypotension refractory to fluids is the signal.
• ARDS — emergency: Neutrophil-mediated alveolar-capillary injury → protein-rich pulmonary edema with bilateral infiltrates and refractory hypoxemia.
• Fistula and sinus tract formation: Inflammation burrows along tissue planes, classically in Crohn disease and chronic osteomyelitis (with a draining sinus).

Complications of chronic inflammation

• AA (secondary) amyloidosis: Chronic elevation of serum amyloid A → nephrotic-range proteinuria with apple-green birefringence on Congo red; suspect in long-standing rheumatoid arthritis, IBD or chronic infection.
• Anemia of chronic disease: Hepcidin-mediated iron sequestration; low serum iron, low TIBC, normal or high ferritin.
• Fibrosis with organ failure: Cirrhosis, IPF, constrictive pericarditis, and hydronephrosis from retroperitoneal fibrosis.
• Malignant transformation: Chronic mucosal injury and regeneration increase mutational burden — colorectal carcinoma in ulcerative colitis, adenocarcinoma in Barrett esophagus, squamous cell carcinoma arising in a chronic wound or burn scar (Marjolin ulcer).

Complications of repair

• Deficient scar → wound dehiscence and evisceration — emergency requiring operative closure; risk rises with obesity, steroids and increased intra-abdominal pressure.
• Excessive scar → hypertrophic scar (within wound borders, may regress) versus keloid (extends beyond borders, recurs after excision).
• Contracture: Myofibroblast-driven, classic after burns and across joints; limits range of motion.
• Peritoneal adhesions → small bowel obstruction, the leading cause after prior abdominal surgery.

Treatment-related

• Glucocorticoids: impaired collagen synthesis → dehiscence and infection masked by suppressed fever; also hyperglycemia, osteoporosis, adrenal suppression.
• TNF inhibitors: reactivation of latent TB and endemic fungi — emergency if disseminated.
• NSAIDs: peptic ulceration and AKI.
• Nintedanib/pirfenidone: diarrhea, hepatotoxicity, and photosensitivity (pirfenidone).

• Neutrophils dominate the first 24 hours; macrophages by day 2–3; granulation tissue by day 3–5; collagen remodeling for months. A stem giving a time interval is testing this timeline, and the wound recovers only a fraction — classically cited as roughly 70–80% — of the original tensile strength, never 100%.
• Type III collagen is laid down first, then replaced by type I during remodeling by zinc-dependent matrix metalloproteinases. Zinc deficiency stalls remodeling; vitamin C deficiency blocks prolyl/lysyl hydroxylation (perifollicular hemorrhage, bleeding gums); copper deficiency blocks lysyl oxidase cross-linking.
• Caseating granuloma = TB or endemic fungi; non-caseating = sarcoidosis, Crohn disease, berylliosis, foreign body, cat-scratch (which is actually stellate/suppurative). The single best next step when a granuloma is found is stains and culture for mycobacteria and fungi before calling it sarcoidosis — sarcoidosis is a diagnosis of exclusion under the ATS/ERS/WASOG guideline.
• IFN-γ from Th1 cells drives epithelioid transformation; TNF-α maintains the granuloma. This is why anti-TNF therapy reactivates latent TB and why latent TB screening precedes infliximab.
• The adhesion cascade in order: selectins (rolling) → integrins/ICAM-1 (adhesion) → PECAM-1 (transmigration). Leukocyte adhesion deficiency type 1 = delayed umbilical separation, marked leukocytosis, no pus; Chédiak-Higashi = giant granules, albinism, neuropathy; CGD = negative dihydrorhodamine flow cytometry (or nitroblue tetrazolium), catalase-positive organisms.
• Fever mechanism: LPS → IL-1/TNF → hypothalamic PGE2 → raised set point. Antipyretics work by COX inhibition, not by "cooling" — a distractor.
• Keloid extends beyond the original wound margins and recurs after excision; a hypertrophic scar stays within them and may regress. Do not choose excision alone for a keloid; intralesional corticosteroid is the standard first move.
• Common distractor: assuming persistent fever on antibiotics means resistant organisms. In a patient with an abscess, the answer is source control — drainage — not broader antibiotics.

• Acute inflammation peaks in hours to days; characterized by neutrophil infiltration and exudation
• Chronic inflammation lasts weeks to years; dominated by macrophages, lymphocytes, and fibroblasts
• Granulomas = hallmark of chronic inflammation (caseating vs non-caseating)
• Repair involves angiogenesis, fibroblast proliferation, and collagen deposition; can result in fibrosis or regeneration
• Scar tissue lacks normal architecture and function; myofibroblasts drive contraction

Repair complications: Excessive collagen → hypertrophic scars or keloids; inadequate repair → chronic ulcers; fibrosis in organs → cirrhosis, pulmonary fibrosis

1. 1.Confusing granulomas with necrosis: Granulomas = organized collection of epithelioid macrophages (chronic); necrosis = cell death. Caseating necrosis is suggestive of TB, not all granulomas have necrosis.

1. 1.Assuming all inflammation is harmful: Low-grade inflammation is essential for repair; excessive or chronic inflammation causes tissue damage. NSAIDs impair initial healing by blocking prostaglandins.

1. 1.Ignoring the resolution phase****: Not all acute inflammation progresses to chronic. Complete resolution can occur with clearance of the inciting agent—test asks for timeline and outcome.

• Acute: Remove stimulus (drain abscess, antibiotics, NSAIDs/corticosteroids if excessive)
• Chronic: Address underlying cause (TB therapy, immunosuppression for autoimmunity); anti-TNF agents for granulomatous diseases if refractory
• Repair/Fibrosis: Antifibrotic agents (pirfenidone, nintedanib for IPF); optimize nutrition, physical therapy; minimize recurrent injury