A protective, time-limited biological response (typically 24-72 hours) to tissue injury or infection, characterized by the cardinal signs of rubor (redness), calor (heat), tumor (swelling), dolor (pain), and functio laesa (loss of function). Initiated by tissue-resident immune cells responding to DAMPs or PAMPs, acute inflammation orchestrates vascular changes, immune cell recruitment, pathogen/debris clearance, and critically, its own resolution through specialized lipid mediator class switching. This is not pathology—it is the essential first phase of tissue repair.
Think of acute inflammation as a fire department responding to a house fire. The alarm goes off (DAMPs/PAMPs trigger TLR4), the fire trucks arrive with sirens blaring and lights flashing (vasodilation causes redness and heat), the firefighters smash windows and spray water everywhere (vascular permeability lets fluid and proteins leak into tissue = swelling), and they tear down damaged walls to prevent spread (neutrophils phagocytose debris = pain from chemical mediators). The chaos looks destructive, but it's saving the house.
Now here's the critical part: after 48-72 hours, the fire chief doesn't just leave the mess. The cleanup crew arrives (macrophages), and they switch tools—instead of axes and hoses (prostaglandins, leukotrienes), they bring mops and rebuilding supplies (resolvins, protectins, maresins). This transition from demolition to reconstruction is lipid mediator class switching. If you suppress the initial fire response with NSAIDs—like handcuffing the firefighters—you don't get a fire, but you also don't get proper cleanup. The house smolders indefinitely (chronic inflammation, chronic pain). The acute response isn't the enemy; premature suppression is.
Acute inflammation begins when tissue damage releases intracellular contents (DAMPs: HMGB1, ATP, mitochondrial DNA) or when pathogens present PAMPs (e.g., LPS from gram-negative bacteria). These danger signals bind pattern recognition receptors on tissue-resident cells:
Initiation (0-4 hours):
- DAMPs/PAMPs → TLR4 (or TLR2, TLR3) on resident macrophages and mast cells
- TLR4 activation → MyD88 adapter protein → NF-κB translocation to nucleus
- NF-κB upregulates: COX-2, iNOS, pro-IL-1β, TNF-α, IL-6
- Mast cells degranulate → histamine release
- Histamine → H1 receptors on endothelial cells → vasodilation, increased vascular permeability
- Bradykinin (via kallikrein-kinin system) → B2 receptors → pain, further vasodilation
- Prostaglandin synthesis: COX-2 converts arachidonic acid → PGH2 → PGE2, PGI2 (prostacyclin)
- PGE2 → EP receptors → fever (via hypothalamic PGE2), pain sensitization, further vasodilation
- Leukotrienes: 5-LOX converts arachidonic acid → LTB4 (neutrophil chemoattractant), LTC4/D4/E4 (vascular permeability)
Leukocyte recruitment (4-48 hours):
graph TD
A["Endothelial activation by TNF-α/IL-1β"] --> B[E-selectin & P-selectin expression]
B --> C[Neutrophil rolling along endothelium]
C --> D[ICAM-1/VCAM-1 expression]
D --> E[Firm neutrophil adhesion]
E --> F[Diapedesis via PECAM-1]
F --> G[Chemotaxis to LTB4, IL-8, C5a gradient]
G --> H[Neutrophil phagocytosis & ROS production]
H --> I[Neutrophil apoptosis at 24-48h]
I --> J[Monocyte recruitment via MCP-1/CCL2]
J --> K["Monocyte → Macrophage differentiation"]
K --> L[Efferocytosis of apoptotic neutrophils]
Resolution phase (48-72 hours):
- Lipid mediator class switching: COX-2 becomes acetylated by aspirin OR S-nitrosylated by NO
- Acetylated COX-2 produces 15-epi-lipoxins (aspirin-triggered lipoxins, ATLs)
- 15-LOX (activated in resolving macrophages) converts DHA → resolvins (RvD1-6), protectins (PD1)
- 15-LOX converts EPA → resolvins (RvE1-3)
- 12-LOX produces maresins (MaR1, MaR2) from DHA
- SPMs bind specific receptors: RvD1 → ALX/FPR2 and GPR32; RvE1 → ChemR23 (ERV1)
- SPM signaling → stops neutrophil recruitment, promotes neutrophil apoptosis, enhances macrophage efferocytosis
- Macrophages shift from M1 (pro-inflammatory) to M2 (pro-resolving) phenotype
- IL-10, TGF-β secretion → dampens NF-κB, suppresses further cytokine production
- Resolution complete by 72-96 hours in uncomplicated injury
Acute inflammation represents a paradigm shift in cPNI practice: it is not the enemy. The Parisien et al. (2022) study in Science Translational Medicine demonstrated that NSAID use during acute pain (0-72 hours post-injury) increases the odds of developing chronic pain (Logâ‚‚OR > 0 for ibuprofen, diclofenac, naproxen across multiple cohorts). Conversely, higher neutrophil percentage at baseline (reflecting robust acute inflammatory capacity) was protective against chronic pain development (Logâ‚‚OR < 0).
This connects directly to Metamodel 0 (evolutionary mismatch): our "take ibuprofen for everything" culture suppresses a conserved, adaptive response that evolved over millions of years. The acute inflammatory cascade—fever, swelling, pain—are not symptoms to eliminate but signals of active repair. Suppressing COX-2 blocks prostaglandin production, which paradoxically prevents the later COX-2 acetylation needed to produce aspirin-triggered lipoxins and initiate resolution.
Clinical thresholds:
- IL-6 peaks at 24-48 hours (typically 20-100 pg/mL in acute injury; >200 pg/mL suggests severe trauma or infection)
- Neutrophil percentage: normal acute response shows 60-80% neutrophils at 24 hours
- CRP rises 6-12 hours post-injury, peaks at 48 hours (typically 10-150 mg/L in uncomplicated acute inflammation; >200 mg/L suggests infection)
- Body temperature: fever 38-39°C is protective (enhances neutrophil function, accelerates resolution)
Intervention strategy:
- First 72 hours: Avoid NSAIDs unless absolutely necessary (e.g., anaphylaxis, life-threatening fever). Support acute inflammation with adequate protein (1.5-2 g/kg for tissue repair), omega-3 fatty acids (2-4 g/day EPA+DHA to ensure SPM substrate availability), hydration, rest.
- 48-72 hours onward: Focus on resolution—SPM supplementation (if available), continued omega-3s, turmeric/curcumin (enhances SPM production via 15-LOX), bromelain (proteolytic support for debris clearance).
- Patient education: reframe fever, swelling, and pain as "healing in progress" rather than "damage to suppress."
- Duration: 24-72 hours in uncomplicated tissue injury; persistent beyond 7 days suggests failed resolution or chronic stimulus
- Cardinal signs: rubor (redness from vasodilation), calor (heat from increased blood flow + prostaglandin-induced fever), tumor (swelling from vascular permeability), dolor (pain from bradykinin, PGE2, substance P), functio laesa (loss of function protects injured tissue)
- Neutrophils arrive first (peak at 24-48 hours), lifespan in tissue 8-20 hours before apoptosis
- Monocytes recruited at 24-48 hours, differentiate into macrophages that persist 3-7 days
- COX-2 induction peaks at 6-12 hours, responsible for PGE2 production (NOT constitutive COX-1)
- Lipid mediator class switching occurs at 48-72 hours: prostaglandins/leukotrienes → resolvins/protectins/maresins
- NSAID use in first 72 hours increases chronic pain risk by suppressing protective inflammation and delaying resolution
- Higher baseline neutrophil percentage (>60%) correlates with successful pain resolution
- Fever up to 39°C is protective: increases neutrophil chemotaxis, phagocytic activity, and bacterial killing
- Omega-3 index <4% predicts poor resolution capacity; optimal >8% for robust SPM production
- NSAIDs — suppress COX-2, blocking both prostaglandin production (intended) and later aspirin-triggered lipoxin synthesis (unintended consequence that prevents resolution), increasing chronic pain risk
- chronic pain — develops when acute inflammation is prematurely suppressed, preventing proper debris clearance, neutrophil apoptosis, and SPM-mediated resolution
- chronic inflammation — failed acute inflammation resolution due to insufficient SPMs, persistent stimulus (PAMPs/DAMPs), or NSAID interference with lipid mediator class switching
- neutrophils — first responders (arrive 4-24 hours), clear debris via phagocytosis and NETosis, their apoptosis at 24-48 hours triggers macrophage efferocytosis and resolution phase
- macrophages — arrive 24-48 hours post-injury, efferocytose apoptotic neutrophils, switch from M1 to M2 phenotype to initiate repair and SPM production
- prostaglandins — COX-2-derived mediators (PGE2, PGI2) essential for vasodilation, fever, pain sensitization during acute phase; substrate for later lipoxin production
- COX-2 — inducible enzyme upregulated by NF-κB, produces PGE2 during acute phase, later acetylated/S-nitrosylated to produce aspirin-triggered lipoxins for resolution
- specialized pro-resolving mediators — resolvins, protectins, maresins produced at 48-72 hours via 15-LOX and 12-LOX from omega-3 fatty acids, actively terminate inflammation
- resolvins — D-series (from DHA) and E-series (from EPA) SPMs that stop neutrophil recruitment, enhance macrophage efferocytosis, reduce pain via TRPV1 inhibition
- lipid mediator class switching — critical transition at 48-72 hours from arachidonic acid-derived pro-inflammatory mediators to omega-3-derived pro-resolving mediators
- 15-LOX — enzyme activated in resolving macrophages, converts DHA to resolvins (RvD1-6) and protectins, converts EPA to RvE1-3
- omega-3 fatty acids — EPA and DHA are essential substrates for SPM biosynthesis; inadequate omega-3 status (<4% omega-3 index) impairs resolution
- mast cells — tissue-resident sentinels that degranulate upon TLR4 activation, releasing histamine, heparin, and TNF-α to initiate vascular changes
- histamine — causes vasodilation (via H1 receptors on smooth muscle) and increased vascular permeability (via H1 on endothelial cells), producing rubor and tumor
- bradykinin — mediates dolor (pain) via B2 receptor activation on nociceptors, also causes vasodilation and increased permeability
- wound healing — acute inflammation is the essential inflammatory phase (days 0-3), followed by proliferative phase (days 3-21) and remodeling (months); suppressing acute inflammation delays all subsequent phases
- tissue repair — requires acute inflammation to clear debris, deposit provisional matrix (fibrin), and recruit fibroblasts; premature NSAID use impairs collagen deposition
- TLR4 — primary pattern recognition receptor for DAMPs (HMGB1, heat shock proteins) and PAMPs (LPS), initiates NF-κB signaling cascade
- DAMPs — damage-associated molecular patterns (HMGB1, ATP, mitochondrial DNA, uric acid) released by necrotic cells, trigger sterile acute inflammation
- PAMPs — pathogen-associated molecular patterns (LPS, peptidoglycan, viral RNA) recognized by TLRs to initiate acute inflammation in infection
- NF-κB — master transcription factor activated downstream of TLR4, upregulates COX-2, iNOS, pro-inflammatory cytokines (TNF-α, IL-1β, IL-6)
- IL-6 — pleiotropic cytokine produced during acute inflammation, induces acute phase response (CRP, fibrinogen), fever, and recruits neutrophils; also has pro-resolving effects via IL-6 trans-signaling
- TNF-α — early pro-inflammatory cytokine that activates endothelial cells (upregulates selectins, ICAM-1), induces fever, and primes neutrophils for enhanced ROS production
- C-reactive protein — acute phase protein rising 6-12 hours post-injury, opsonizes pathogens and apoptotic cells for phagocytosis, peak at 48 hours
- fever — adaptive response mediated by PGE2 acting on hypothalamic EP3 receptors, enhances neutrophil activity and accelerates resolution; suppression with NSAIDs/acetaminophen may prolong illness
- efferocytosis — macrophage phagocytosis of apoptotic neutrophils, essential for resolution; triggered by phosphatidylserine exposure on neutrophils and enhanced by RvD1, MaR1
- Module 1 — Introduction to cPNI, evolutionary mismatch, selfish systems
- Module 5 — Inflammation and immune regulation, resolution pharmacology