The immune system's coordinated, biphasic response to tissue damage, pathogens, or danger signals (DAMPs and PAMPs), characterized by vascular changes (vasodilation, increased permeability), leukocytes infiltration, and sequential release of pro-inflammatory and pro-resolving mediators. acute inflammation is a protective, self-limiting process that eliminates threats and initiates wound healing; chronic inflammation represents resolution failure and drives the majority of modern non-communicable diseases through sustained tissue damage and metabolic reprogramming.
Think of inflammation as a city's emergency response to a fire. When the fire alarm sounds (tissue damage detected), fire trucks (neutrophils) arrive within minutes, sirens blaring (cytokines like TNF-α and IL-1β). Roads widen (vasodilation), and barriers open to let emergency vehicles through (increased vascular permeability causing swelling). The fire station calls in reinforcements from neighboring districts (chemokines recruiting more immune cells). Firefighters spray water and chemicals everywhere—yes, they're putting out the fire, but they're also causing water damage to the building (collateral tissue damage).
Now here's the crucial part: once the fire is out, a second crew must arrive—the cleanup and reconstruction team (macrophages producing Specialized pro-resolving mediators (SPMs) like Resolvins and Maresins). This crew actively pumps out the water, repairs the damage, and tells everyone to go home. Without this second phase—the resolution crew—the fire trucks just keep circling, sirens blaring forever, damaging the building more than the original fire did. That's chronic inflammation: emergency responders who never got the memo that the crisis is over, slowly demolishing the neighborhood they're supposed to protect.
Inflammation initiates when sentinel cells—macrophages, Mast cells, dendritic cells, and tissue-resident immune cells—detect danger signals via pattern recognition receptors (TLR4, NOD-Like Receptors, RIG-I-like receptors). This detection triggers:
Initiation Phase (0-6 hours):
- TLR4 activation (by LPS or endogenous DAMPs) → MyD88/TRIF signaling → NF-κB nuclear translocation
- NF-κB drives transcription of pro-inflammatory genes → TNF-α, IL-1β, Interleukin-6, IL-8, COX-2
- TNF-α and IL-1β act on endothelial cells → upregulation of adhesion molecules (E-selectin, P-selectin, ICAM-1, VCAM-1)
- Mast cells degranulate → immediate release of preformed Histamine, Bradykinin, heparin
- COX-2 converts arachidonic acid → PGE2, PGI2, thromboxane A2
- 5-LOX converts arachidonic acid → leukotrienes (LTB4, LTC4, LTD4, LTE4)
Vascular Response:
- Histamine + PGE2 + NO → arteriolar vasodilation (rubor, calor)
- Histamine + bradykinin → increased endothelial gap junctions → plasma protein extravasation → edema (tumor)
- Bradykinin + PGE2 + nerve growth factor → nociceptor sensitization via TRPV1 and TRPA1 channels (dolor)
Cellular Recruitment:
graph TD
A[Tissue Damage/Pathogen] --> B[Sentinel Cell Activation]
B --> C["TNF-α, IL-1β Release"]
B --> D["Chemokine Release: IL-8, MCP-1"]
C --> E[Endothelial Adhesion Molecule Expression]
D --> F[Chemotactic Gradient Formation]
E --> G[Neutrophil Rolling on Selectins]
G --> H[Firm Adhesion via ICAM-1/LFA-1]
H --> I[Transmigration via PECAM-1]
F --> I
I --> J[Neutrophil Arrival 30-60 min]
J --> K[Phagocytosis, ROS, NETs]
K --> L[Monocyte Recruitment 24-48h]
L --> M["Monocyte → Macrophage Differentiation"]
M --> N{Resolution Signal?}
N -->|Yes| O[M2 Polarization, SPM Production]
N -->|No| P["Sustained M1 Activity → Chronic Inflammation"]
O --> Q[Resolution Complete 2-4 weeks]
Acute Phase Response (Systemic):
- IL-6 + IL-1β + TNF-α → hepatic acute phase protein synthesis
- C-reactive protein rises from <1 mg/L to >200 mg/L within 24-48 hours
- Serum amyloid A (SAA), fibrinogen, ferritin, haptoglobin increase
- Albumin, transferrin decrease (negative acute phase proteins)
Resolution Phase (Requires Active Switching):
Chronic Inflammation Mechanisms:
- Persistent pathogen presence or unresolved tissue damage
- Inadequate SPM production (dietary omega-3 deficiency, genetic polymorphisms in 15-LOX)
- obesity-induced metabolic inflammation (metaflammation): adipocyte hypertrophy → hypoxia → HIF-1 → sustained IL-6, TNF-α
- dysbiosis → chronic LPS exposure → continuous TLR4 stimulation
- Ageing-associated inflammaging: cellular senescence → senescence-associated secretory phenotype (SASP) → perpetual cytokine production
Inflammation sits at the mechanistic core of virtually all cPNI practice. Understanding its biphasic nature—initiation followed by mandatory resolution—is essential for avoiding iatrogenic harm and addressing root causes.
Acute Inflammation as Survival Mechanism:
- Necessary for pathogen elimination and wound repair—blocking it impairs healing
- NSAID use in acute injuries delays fracture healing (blocks COX-2-mediated bone formation) and impairs muscle regeneration (prevents satellite cell proliferation)
- Metamodel 0 (evolutionary expectations): acute inflammation evolved in contexts of intermittent injuries/infections, not chronic activation
Chronic Low-Grade Inflammation as Disease Driver:
- Defined as C-reactive protein >3 mg/L persistently or IL-6 >3-5 pg/mL
- Underpins: Type 2 Diabetes (IL-1β pancreatic β-cell toxicity), atherosclerosis (endothelial dysfunction, plaque formation), Cancer (DNA damage, angiogenesis), Alzheimer's Disease (microglial activation, Aβ aggregation), Depression (kynurenine pathway activation via IDO)
- Metamodel 5 (metabolic redistribution): chronic inflammation is energetically expensive—immune system becomes selfish, demanding glucose at expense of brain, muscle
Resolution Failure:
Immunoceptivity and Subjective Experience:
Biomarker Interpretation:
- hsCRP <1 mg/L: low risk; 1-3 mg/L: moderate; >3 mg/L: high inflammatory burden
- IL-6 >10 pg/mL suggests active inflammation; >50 pg/mL seen in sepsis/cytokine storm
- Ferritin elevated without anemia suggests inflammation (acute phase reactant), not iron overload
- Neutrophil-lymphocyte ratio >3 indicates systemic inflammation
Treatment Paradox:
- NSAIDs block COX-2 → prevent prostaglandin synthesis and SPM synthesis → impair both inflammation and resolution
- Aspirin uniquely acetylates COX-2 → converts it to produce aspirin-triggered resolvins (pro-resolution), explaining cardioprotective effects beyond antiplatelet action
- Glucocorticoids suppress initiation but also block resolution—chronic use traps patients in unresolved inflammation upon withdrawal
- The five cardinal signs of Celsus (30 AD): rubor (redness), calor (heat), tumor (swelling), dolor (pain), plus Virchow's (1858) functio laesa (loss of function)
- neutrophil chemotaxis to injury site occurs within 30-60 minutes; monocytes arrive 24-48 hours later
- C-reactive protein has a half-life of 19 hours—rapid rise (6-8 hours post-injury) and fall make it ideal for monitoring acute inflammation
- Normal acute inflammatory resolution completes within 2-4 weeks; persistence beyond 6 weeks indicates chronicity
- IL-6 levels peak 4-6 hours post-injury at 100-1000× baseline, then decline as resolution begins
- PGE2 induces fever by acting on EP3 receptors in the hypothalamic preoptic area, raising thermoregulatory set-point by 2-3°C
- Chronic inflammation increases basal metabolic rate by 20-40% due to immune cell energy demands (selfish immune system)
- TNF-α and IL-1β synergistically induce endothelial adhesion molecules within 2-4 hours (VCAM-1, ICAM-1, E-selectin)
- Omega-3 index <4% correlates with impaired SPM synthesis and prolonged inflammation; >8% optimal for resolution
- NF-κB activation is biphasic: rapid (minutes) for immediate response genes, sustained (hours-days) in chronic inflammation via positive feedback loops
- resolution requires active lipid mediator class switching—blocking COX-2 prevents both PGE2 (pro-inflammatory) and resolvin (pro-resolution) synthesis
- acute inflammation — protective, self-limiting form of inflammation essential for pathogen clearance
- chronic inflammation — pathological persistent inflammation representing resolution failure
- chronic low-grade inflammation — sustained elevation of inflammatory markers driving metabolic and neurodegenerative diseases
- Cytokines — soluble protein mediators orchestrating inflammatory cell recruitment and activation
- TNF-α — proximal pro-inflammatory cytokine triggering endothelial activation and fever
- IL-1β — pyrogenic cytokine activating NF-κB and inducing acute phase response
- Interleukin-6 — pleiotropic cytokine driving hepatic acute phase protein synthesis and metabolic reprogramming
- C-reactive protein — acute phase protein biomarker; >3 mg/L indicates chronic inflammation
- Prostaglandins — COX-2-derived lipid mediators producing vasodilation, fever, and pain sensitization
- leukotrienes — 5-LOX-derived lipid mediators causing bronchoconstriction, vascular permeability, and neutrophil chemotaxis
- Specialized pro-resolving mediators (SPMs) — lipid mediators actively terminating inflammation and promoting tissue repair
- Resolvins — DHA/EPA-derived SPMs binding ALX-FPR2 to dampen NF-κB signaling
- Protectins — DHA-derived SPMs protecting neural tissue and promoting resolution
- Maresins — macrophage-derived SPMs enhancing efferocytosis and M2 polarization
- resolution of inflammation — active biochemical process terminating immune responses and restoring homeostasis
- Lipid mediator class switching — COX-2 substrate shift from arachidonic acid to EPA/DHA enabling SPM synthesis
- NF-κB — master transcription factor driving expression of pro-inflammatory genes (TNF-α, IL-1β, COX-2)
- COX-2 — inducible enzyme converting arachidonic acid to prostaglandins; substrate for SPM synthesis when acetylated
- TLR4 — pattern recognition receptor detecting LPS and DAMPs, initiating inflammatory signaling
- neutrophils — first-responder leukocytes arriving 30-60 min post-injury for phagocytosis and ROS production
- macrophages — tissue-resident phagocytes coordinating both initiation (M1) and resolution (M2) phases
- M1 macrophages — pro-inflammatory phenotype producing TNF-α, IL-1β, ROS
- M2 macrophages — pro-resolving phenotype producing IL-10, TGF-β, SPMs
- PAMPs — pathogen-associated molecular patterns triggering infectious inflammation via TLRs
- DAMPs — damage-associated molecular patterns triggering sterile inflammation from cell death
- metaflammation — metabolic inflammation driven by obesity, insulin resistance, and adipose tissue dysfunction
- neuroinflammation — CNS-specific inflammation involving microglia, astrocytes, and BBB disruption
- inflammaging — age-related chronic low-grade inflammation from cellular senescence and immunosenescence
- sickness behaviour — adaptive behavioral response to peripheral inflammation mediated by IL-1β, IL-6 brain signaling
- Cortex Insularis — brain region integrating inflammatory signals into conscious interoceptive experience
- Depression — often driven by chronic inflammation activating IDO and kynurenine pathway
- obesity — creates chronic inflammatory state via adipocyte hypoxia, macrophage infiltration, free fatty acid release
- dysbiosis — gut microbiome imbalance increasing LPS translocation and systemic inflammation
- gut permeability — intestinal barrier dysfunction allowing bacterial products to drive systemic inflammation
- stress — chronic psychological stress impairs glucocorticoid signaling, promoting inflammation
- sleep deprivation — disrupts SPM synthesis and increases pro-inflammatory cytokine production
- sedentary behavior — reduces lymphatic flow and impairs inflammatory resolution
- Vagus nerve — parasympathetic anti-inflammatory pathway via acetylcholine and α7 nicotinic receptors
- Efferocytosis — macrophage clearance of apoptotic neutrophils, essential for resolution
- omega-3 fatty acids — EPA and DHA are SPM precursors; deficiency impairs resolution
- arachidonic acid — omega-6 fatty acid precursor to pro-inflammatory prostaglandins and leukotrienes
- Type 2 Diabetes — chronic inflammation (IL-1β) induces pancreatic β-cell dysfunction
- atherosclerosis — inflammatory disease of arterial walls driven by oxidized LDL and macrophage foam cells
- autoimmune disease — inappropriate inflammatory response to self-antigens, often involving chronic inflammation
- Module 1 — Introduction to inflammation as core immune process
- Module 4 — Inflammation in chronic disease pathogenesis
- Module 5 — Metabolic aspects of inflammation and resolution