Acute Inflammation — Mediators and Cellular Events

Vascular Events (Hemodynamic Phase)

• Transient vasoconstriction (seconds) followed by sustained arteriolar and capillary bed vasodilatation, primarily mediated by histamine, nitric oxide (NO), and prostaglandin I2 (PGI2); increases blood flow causing erythema and warmth
• Increased vascular permeability in post-capillary venules (primary site of fluid extravasation) via:
• Endothelial cell contraction creating intercellular gaps (immediate transient response, 15-30 minutes)—mediated by histamine, bradykinin, leukotrienes (LTC4, LTD4, LTE4), substance P, and complement components (C3a, C5a)
• Delayed prolonged leakage (2-8 hours, peaks at 24-48 hours)—associated with direct endothelial injury from reactive oxygen species (ROS), proteases, and leukocyte-derived mediators
• Endothelial injury leakage (hours to days)—results from severe thermal, chemical, or ischemic injury with loss of structural integrity
• Blood stasis and hemoconcentration result from fluid loss; increased blood viscosity and venous stasis facilitate leukocyte margination (concentration of cells at vessel periphery)

Exudation and Plasma Protein Movement

• Protein-rich fluid exudation (plasma exudate with protein concentration >2.5 g/dL) accumulates in interstitial space, increasing interstitial oncotic pressure and perpetuating fluid extravasation
• Fibrinogen conversion to fibrin creates fibrin clots that:
• Wall off injury site and prevent pathogen dissemination
• Provide scaffold for cell migration and tissue repair
• Generate fibrin-derived peptides (D-dimer fragments) with chemotactic properties
• Plasma protein loss includes coagulation factors, complement proteins, and antibodies

Cellular Events (Leukocyte Recruitment)

• Margination: Leukocytes (primarily neutrophils) migrate to vessel periphery due to blood stasis; selectins on endothelium (E-selectin, P-selectin) and leukocytes (L-selectin) mediate initial tethering
• Rolling: Selectin-mediated weak interactions allow rapid cell movement along endothelial surface; enables sampling of endothelial activation signals
• Firm adhesion:
• Integrin-ICAM interaction is critical—leukocyte integrins (β2-integrins: LFA-1, Mac-1) bind endothelial ICAM-1 and ICAM-2; integrin activation requires intermediate signaling (upregulation via chemokine receptor engagement)
• VCAM-1/VLA-4 interaction particularly important in lymphocyte and eosinophil recruitment
• Mediated by PAF (platelet-activating factor), C5a, TNF-α, IL-1, and chemokines (IL-8/CXCL8, MCP-1/CCL2, GRO-α/CXCL1)
• Transmigration (diapedesis):
• Leukocytes squeeze between endothelial cells (primarily paracellular route)
• PECAM-1 (CD31) on adjacent endothelial cells and leukocytes facilitates homophilic interaction
• Most leukocytes transmigrate at post-capillary venules; some via capillaries
• Followed by chemotaxis—directional migration toward chemotactic stimuli concentration gradient

Leukocyte Activation and Functions

Neutrophil-mediated phagocytosis and bacterial killing

• Recognition and opsonization: Bacteria coated with IgG, complement C3b/iC3b, or directly recognized via pattern recognition receptors (TLRs, formyl-peptide receptors) on neutrophil surface
• Phagocytosis: Neutrophil engulfment of pathogen into phagosome; Fc receptors (FcγRIIa) and complement receptors (CR1, CR3) drive internalization
• Phagolysosome formation: Fusion with azurophilic (primary) and specific (secondary) granules releases:
• Myeloperoxidase (MPO): generates hypochlorous acid (HOCl), the most potent antimicrobial agent
• Serine proteases: cathepsin G and elastase degrade virulence factors
• Lactoferrin and lysozyme: antimicrobial enzymes
• Oxidative burst (respiratory burst):
• NADPH oxidase activation generates superoxide anion (O2•−) → dismutation to H2O2 → conversion to HOCl by MPO
• Highly reactive oxygen species (ROS) kill microorganisms but also cause tissue damage
• Defects in NADPH oxidase cause chronic granulomatous disease (CGD) with recurrent infections
• Degranulation: Exocytosis of granule contents into extracellular space and phagolysosome; also releases:
• Proteolytic enzymes (elastase, collagenase, gelatinase)—degrade extracellular matrix and tissue proteins
• Cationic proteins—directly antimicrobial
• Contributes to tissue damage in inflammatory conditions

Chemical Mediators of Inflammation

Vasoactive amines

• Histamine: Released from mast cell and basophil granules by IgE cross-linking, complement (C3a, C5a), substance P; causes vasodilatation, increased vascular permeability; short-lived (inactivated by histaminase)
• Serotonin: Released from platelet granules; mild vasoactive effects

Arachidonic acid metabolites (eicosanoids)

• Prostaglandins (PGE2, PGI2): Vasodilatation, pain, potentiate edema formation; inhibited by NSAIDs and corticosteroids
• Thromboxane A2 (TXA2): Platelet activation, vasoconstriction
• Leukotrienes (LTC4, LTD4, LTE4): Potent vasoconstriction, increased vascular permeability, bronchoconstriction, leukocyte recruitment; act through cysteinyl leukotriene receptors

Complement system mediators

• C3a and C5a (anaphylatoxins): Mast cell degranulation, leukocyte recruitment, opsonization, direct bactericidal activity
• C5a: Most potent complement-derived chemotactic agent; also promotes leukocyte activation and ROS production
• MAC (C5b-9): Forms pores in cell membranes, causes cell lysis; at sublytic doses activates endothelial cells

Kinins

• Bradykinin: Generated from high-molecular-weight kininogen by factor XIIa (contact activation); causes vasodilation, increased permeability, pain; inactivated by kininase II (ACE)
• Rapidly degraded, short half-life

Cytokines

• TNF-α: Synergistic with IL-1; activates endothelium, increases ICAM-1 and VCAM-1, promotes leukocyte recruitment; pyrogenic (fever); promotes shock in sepsis
• IL-1: Similar functions to TNF-α; promotes prostaglandin and leukocyte synthesis
• IL-6: Acute phase protein production
• IL-8 (CXCL8) and other chemokines (MCP-1/CCL2, GRO-α/CXCL1): Direct leukocyte chemotaxis and activation

Other mediators

• PAF (platelet-activating factor): Potent leukocyte activator, increases vascular permeability, promotes platelet activation and aggregation
• Substance P: Neuropeptide; increases vascular permeability, promotes vasodilation
• Lipid mediators: Derivatives of arachidonic acid
• ROS and RNS (reactive nitrogen species): Killing organisms but causing tissue damage
• Proteolytic fragments: Fibrin-derived peptides, complement-derived peptides, bacterial products (N-formyl-methionine peptides)

Morphological Pattern of Acute Inflammation

• Acute exudative inflammation: Characterized by vascular changes and neutrophilic exudate
• Serous inflammation: Watery exudate with protein concentration 2-3 g/dL (e.g., initial burn response, viral hepatitis)
• Fibrinous inflammation: Extensive fibrin deposition, often with severe endothelial injury (e.g., acute rheumatic fever, vasculitis, bacterial pneumonia with fibrinopurulent exudate)
• Purulent inflammation: Pus formation (dead neutrophils, bacteria, tissue debris, fibrin); indicates bacterial infection
• Hemorrhagic inflammation: RBC extravasation; indicates severe vascular injury

Infectious Agents

• Bacteria: Most common cause; disseminated pyogenic pathogens (Staphylococcus aureus, Streptococcus pyogenes, Escherichia coli, Neisseria meningitidis, Pseudomonas aeruginosa)
• Viruses: Trigger acute inflammatory response; recruit lymphocytes and macrophages
• Fungi, parasites: Elicit acute inflammation with varying cellular components

Tissue Injury (Non-infectious)

• Traumatic injury: Physical trauma, crush injuries, lacerations with hemorrhage
• Thermal injury: Burns (3rd and 4th degree) cause massive necrosis and acute inflammation
• Chemical injury: Corrosive substances, acids, bases
• Radiation injury: UV light, ionizing radiation causing cell death
• Ischemia-reperfusion injury: Post-MI, post-surgical complications; ROS generation

Immune-Mediated Causes

• Type III hypersensitivity: Immune complex deposition (e.g., SLE, serum sickness)
• Type IV hypersensitivity: T-cell mediated (e.g., delayed contact dermatitis)
• Autoimmune diseases: Rheumatoid arthritis, autoimmune gastritis

Physical and Chemical Irritants

• Asbestos inhalation: Irritant particles triggering pulmonary inflammation
• Silica dust: Occupational lung disease
• Foreign body reaction: Splinters, sutures, surgical materials

Other Triggers

• Complement activation: Via classical (immune complex), alternative, or lectin pathways
• Mast cell degranulation: IgE-mediated (immediate hypersensitivity) or direct (physical trauma, temperature extremes)
• Metabolic factors: Gout (monosodium urate crystals), pseudogout (calcium pyrophosphate crystals)
• Neoplasia: Tumors trigger acute inflammation through necrosis and immune activation

Local Signs and Symptoms (Cardinal Signs of Inflammation)

• Calor (Heat): Increased local blood flow and metabolic activity; palpable warmth, may be systemic if severe
• Rubor (Redness): Vasodilatation and increased blood perfusion; erythema visible in skin/mucous membranes
• Tumor (Swelling/Edema): Plasma exudation into interstitium; palpable swelling, increased tissue turgor; can compromise function if large (e.g., airway edema in epiglottitis)
• Dolor (Pain): Stimulation of nociceptors by:
• Prostaglandins (PGE2, especially potentiated by bradykinin)
• Substance P
• Cytokines (IL-1, TNF-α)
• Mechanical pressure from edema
• Functio Laesa (Loss of Function): Results from swelling, pain, and tissue destruction; e.g., immobility in septic arthritis, difficulty swallowing in acute pharyngitis

Systemic Manifestations (in significant acute inflammation)

• Fever: Mediated by IL-1 and TNF-α acting on hypothalamic thermoregulatory centers; presents with chills followed by elevated temperature
• Malaise and fatigue: Cytokine-mediated (TNF-α, IL-1)
• Leukocytosis:
• Shift to immature forms (left shift): increased bands and myelocytes on differential
• Mediated by G-CSF (granulocyte-colony stimulating factor), TNF-α, IL-6
• Neutrophil count often >15,000/μL in acute bacterial infection
• Leukemoid reaction (severe leukocytosis >50,000) can mimic leukemia
• Tachycardia and tachypnea: Compensatory response to fever, pain, and possible hypoxemia
• Elevated acute phase proteins:
• C-reactive protein (CRP): Rises within 6-8 hours, peaks at 48-72 hours
• Erythrocyte sedimentation rate (ESR): Slower rise; nonspecific

Organ-Specific Presentations

• Acute bacterial meningitis: Fever, neck stiffness, photophobia, altered mental status; CSF shows elevated protein and WBC (predominantly neutrophils), decreased glucose
• Acute pneumonia: Cough, fever, dyspnea, pleuritic chest pain; consolidation on CXR; sputum may be purulent
• Acute appendicitis: Periumbilical pain migrating to RLQ, rebound tenderness, fever; elevated WBC
• Acute pyelonephritis: Flank pain, fever, dysuria, pyuria with WBC casts and bacteria on UA

Physical Examination Findings

• Localized erythema and edema: Over site of infection or injury
• Purulent drainage: Indicates pyogenic bacterial infection with abscess formation
• Regional lymphadenopathy: Activation and proliferation of draining lymph nodes
• Vital sign abnormalities: Fever, tachycardia, hypotension (in sepsis)

Laboratory and Imaging Correlates

• Complete blood count: Leukocytosis with left shift (immature forms)
• Inflammatory markers: CRP, ESR elevation
• Culture and sensitivity: Identification of causative organism; guides antibiotic selection
• Imaging:
• Plain radiography: Consolidation (pneumonia), free air (perforation), air-fluid levels
• Ultrasound/CT: Fluid collections, abscesses, organ enlargement
• MRI: Soft tissue infections, bone marrow edema (osteomyelitis)

Histological Features

Acute inflammatory infiltrate

• Predominance of neutrophilic polymorphonuclear leukocytes (PMNs):
• Numerous segmented nuclei and band forms in tissue and circulating blood
• Nuclear debris from leukocyte apoptosis and necrosis
• Reflects recent recruitment (neutrophils appear within hours,

Immediate stabilization (when systemic)

• Sepsis/septic shock: the Surviving Sepsis Campaign recommends obtaining cultures then giving broad-spectrum antimicrobials — immediately (within one hour) for septic shock — plus intravenous crystalloid resuscitation (at least 30 mL/kg for hypoperfusion or lactate elevation) and norepinephrine as first-line vasopressor titrated to a mean arterial pressure of 65 mmHg. Balanced crystalloid is favored over normal saline.
• Anaphylaxis (mast-cell histamine/leukotriene surge): epinephrine 0.3 mg IM into the anterolateral thigh is first-line per the AAAAI/ACAAI Joint Task Force; antihistamines and glucocorticoids are adjuncts only and never delay epinephrine.

First-line anti-inflammatory therapy

• Non-selective COX inhibitors (NSAIDs), e.g. ibuprofen — block prostaglandin synthesis, reducing vasodilation, edema potentiation, fever, and PGE2-mediated nociceptor sensitization.
• Glucocorticoids, e.g. prednisone — induce annexin A1, inhibiting phospholipase A2 and thereby both COX and LOX arms, and suppress NF-κB-driven TNF-α/IL-1/IL-8 transcription.
• For crystal-induced acute inflammation, the ACR gout guideline endorses NSAIDs, systemic or intra-articular glucocorticoids, or colchicine (which blocks microtubule-dependent neutrophil chemotaxis) as equally acceptable first-line options.

Escalation and definitive management

• Source control is definitive: incision and drainage of abscess, debridement of necrotic tissue, appendectomy, removal of infected hardware or catheters. Antibiotics do not penetrate undrained pus.
• Targeted biologics (anti-TNF, IL-1 antagonists, IL-6 receptor blockade) are reserved for defined immune-mediated disease, not undifferentiated inflammation.

Contraindicated / avoid

• Aspirin in children with febrile viral illness — Reye syndrome.
• NSAIDs in advanced CKD, active peptic ulcer disease, or volume depletion (prostaglandin-dependent afferent arteriolar tone).
• Glucocorticoids as monotherapy in undrained infection — they blunt neutrophil recruitment and mask localizing signs.

Local complications of the inflammatory response

• Abscess: liquefactive necrosis walled off by fibrin and granulation tissue; neutrophil proteases and ROS destroy the parenchyma. Signaled by persistent fever and leukocytosis despite appropriate antibiotics, with a rim-enhancing fluid collection on CT. Requires drainage.
• Ulceration: loss of surface epithelium when the exudative process is mucosal or cutaneous; bleeding or perforation follows.
• Fistula and sinus tract formation: an abscess burrows along tissue planes to an epithelial surface; enteric or purulent drainage from an unexpected site is the clue.
• Fibrosis and stricture: unresolved exudate that is not cleared undergoes organization — granulation tissue and collagen replace it. In the pleura this yields a fibrothorax with restrictive physiology; in the pericardium, constrictive pericarditis.
• Progression to chronic inflammation: persistent stimulus shifts the infiltrate from neutrophils to macrophages, lymphocytes, and plasma cells.

Systemic complications — emergencies

• Septic shock: TNF-α/IL-1-driven NO overproduction causes profound vasodilation; hypotension refractory to fluids plus lactate elevation defines it.
• ARDS: neutrophil-mediated alveolar–capillary injury producing protein-rich exudate; bilateral opacities with hypoxemia not explained by heart failure.
• DIC: tissue factor expression on cytokine-activated endothelium and monocytes; thrombocytopenia, prolonged PT/aPTT, low fibrinogen, elevated D-dimer.
• Airway or compartment compression from edema: epiglottic swelling or limb compartment syndrome — decompression is time-critical.
• Secondary (AA) amyloidosis: chronic serum amyloid A elevation; proteinuria is the presenting sign.

Treatment-related complications

• NSAIDs: mucosal PGE2 loss → GI bleeding; afferent arteriolar constriction → AKI; aspirin in children → Reye syndrome.
• Glucocorticoids: hyperglycemia, HPA axis suppression, opportunistic infection, and demargination neutrophilia that mimics infection — a classic distractor.
• Antibiotics: Clostridioides difficile colitis; vancomycin is dosed to a 24-hour AUC/MIC of 400–600 per the IDSA/ASHP consensus, with AKI risk at higher exposures.
• Anti-TNF biologics: reactivation of latent tuberculosis — CDC/ACR require IGRA or TST screening before initiation.

• The four chemotactic agents for neutrophils: C5a, LTB4, IL-8 (CXCL8), and bacterial N-formyl-methionine peptides. C5a is the most potent complement-derived chemoattractant; C3a is an anaphylatoxin but a weak chemotaxin — that distinction is the tested distractor.
• The post-capillary venule is the site of both fluid leakage and leukocyte transmigration. If a stem asks where edema forms, this is the answer, not the capillary or arteriole.
• Exudate versus transudate: exudate is protein-rich (>2.5 g/dL) with high LDH and high specific gravity, from increased permeability; transudate is protein-poor, from altered Starling forces. For pleural fluid, apply Light's criteria.
• Leukocyte adhesion deficiency type 1: CD18/β2-integrin defect → no firm adhesion. Look for delayed umbilical cord separation, recurrent bacterial infections without pus, and marked peripheral leukocytosis. LAD type 2 is a sialyl-Lewis X (selectin ligand) defect affecting rolling.
• Chronic granulomatous disease: NADPH oxidase defect. The best next diagnostic step is the dihydrorhodamine 123 flow cytometry test, which has replaced nitroblue tetrazolium. Catalase-positive organisms — S. aureus, Serratia, Burkholderia, Nocardia, Aspergillus.
• Chédiak–Higashi: LYST mutation impairs phagolysosome fusion → giant cytoplasmic granules, albinism, neuropathy.
• Fever mechanism: IL-1 and TNF-α induce hypothalamic COX-2 → PGE2 → raised set point. Antipyretics work by COX inhibition, which is why the pathway, not the cytokine, is the drug target.
• Timing on histology: neutrophils dominate the first ~24 hours; macrophages predominate by 2–3 days; granulation tissue appears around days 3–5. A stem giving a lesion age is testing this sequence.