Stimuli of Acute Inflammation: Infection, Necrosis, Trauma, Toxins, Allergy and Crystal Injury

Learning outcomes

• List major stimuli of acute inflammation and classify them as infectious or sterile.
• Explain how PAMPs, DAMPs, immune complexes, complement, crystals and toxins activate inflammatory pathways.
• Recognise the clinical clues and emergency risks associated with each stimulus.
• Distinguish the stimulus from the downstream morphologic pattern.
• Choose targeted investigations and initial management priorities.

1. Stimulus versus response

Term	Meaning	Example
Stimulus/trigger	The initiating event that disturbs tissue homeostasis.	Bacterial invasion, infarction or a burn.
Recognition signal	PAMP, DAMP, immune complex or crystal sensed by a receptor.	Lipopolysaccharide activates TLR4.
Inflammatory response	Vascular change, mediator release and leukocyte recruitment.	Neutrophilic exudate in bacterial pneumonia.
Outcome	Resolution, abscess, fibrosis, chronic inflammation or organ failure.	Scar after myocardial infarction.

2. Infectious stimuli

2.1 Bacteria

Bacterial surfaces and toxins trigger TLRs, NOD-like receptors and inflammasomes. Pyogenic organisms produce neutrophil-rich suppuration; invasive organisms may cause necrosis, thrombosis and sepsis.

• Extracellular bacteria: complement, antibodies and neutrophils dominate.
• Intracellular bacteria: macrophages, NK cells and T cells become important.
• Exotoxins: direct cytotoxicity, pore formation or systemic neurotoxicity.
• Endotoxin: TLR4-driven cytokines, endothelial activation and shock.

2.2 Viruses

Viral RNA/DNA is detected by endosomal or cytosolic sensors. Type I interferons restrict replication; cytotoxic T cells and NK cells kill infected cells. Acute viral inflammation may cause bronchiolitis, hepatitis, encephalitis or myocarditis.

2.3 Fungi

Beta-glucans and mannans activate C-type lectin receptors. Neutrophils are critical for mould killing, while granulomas contain some deep fungal infections. Invasive fungal disease can produce haemorrhage and infarction.

2.4 Parasites

Helminths stimulate IgE, mast cells and eosinophils; tissue migration can cause urticaria, bronchospasm, obstruction and granulomatous inflammation. Protozoa may cause macrophage or cytotoxic T-cell responses.

2.5 Microbial patterns and acute emergencies

• Sepsis and septic shock.
• Meningitis/encephalitis and raised intracranial pressure.
• Necrotising soft-tissue infection and gas gangrene.
• Severe pneumonia with respiratory failure.
• Invasive fungal disease in neutropenia or immunosuppression.

3. Tissue necrosis and DAMP stimuli

Dead or severely injured cells release DAMPs that produce sterile inflammation. Important settings include:

Necrotic stimulus	Examples	Clinical risks
Ischaemic infarction	Heart, brain, kidney, spleen or bowel.	Reperfusion injury, arrhythmia, oedema, organ failure.
Crush injury	Muscle compression, compartment syndrome.	Rhabdomyolysis, hyperkalaemia, acute kidney injury.
Pancreatic enzyme injury	Acute pancreatitis.	Shock, hypocalcaemia, respiratory/renal failure.
Burns/frostbite	Thermal injury and reperfusion.	Airway oedema, fluid loss, tissue loss.
Traumatic necrosis	Blunt injury, blast, surgery.	Haemorrhage, compartment syndrome, infection.

DAMPs include extracellular ATP, HMGB1, mitochondrial DNA, uric acid, heat-shock proteins and exposed phospholipids. They activate TLRs, NLRP3 inflammasomes and complement even when no organism is present.

4. Physical stimuli

• Heat and burns: protein denaturation, endothelial injury, DAMP release and fluid loss.
• Cold/frostbite: vasoconstriction, ice crystals, thrombosis and reperfusion inflammation.
• Mechanical injury: cell rupture, haemorrhage and oedema after trauma or surgery.
• Radiation: DNA damage, ROS and endothelial injury.
• Electricity/lightning: thermal necrosis, arrhythmia and muscle injury.
• Pressure/barotrauma: tissue disruption and ischaemia during decompression or ventilation.
• Ultraviolet light: keratinocyte damage and sunburn inflammation.

5. Chemical and drug stimuli

Stimulus	Acute inflammatory mechanism	Clue
Corrosive acid/alkali	Direct protein and membrane destruction.	Oropharyngeal burns, dysphagia, airway oedema.
Smoke/irritants	Epithelial injury, ROS and bronchial mediator release.	Cough, wheeze, hypoxia or delayed lung injury.
Paracetamol toxicity	Reactive metabolite, glutathione depletion and hepatocyte necrosis.	Overdose history, later liver failure.
Drug hypersensitivity	IgE, immune complexes or T-cell injury.	Rash, eosinophilia, hepatitis, nephritis or anaphylaxis.
Heavy metals/pesticides	Enzyme inhibition, oxidative stress and cell injury.	Neurologic, gastrointestinal or renal syndrome.

6. Foreign-body stimuli

Splinters, sutures, talc, silica, asbestos, prosthetic debris and keratin trigger inflammation because they cannot be cleared. Macrophages surround the material, fuse into giant cells and release cytokines and ROS. The response may remain local or cause granulomatous fibrosis.

In the emergency setting, remove penetrating material when safe, image retained foreign bodies and consider infection in a draining sinus or non-healing wound.

7. Immune and allergic stimuli

7.1 IgE-mediated allergy

Allergen cross-linking of IgE on mast cells produces histamine, tryptase, prostaglandins and leukotrienes. Urticaria, angioedema, bronchospasm, vomiting and hypotension can develop within minutes.

7.2 Immune complexes

Deposited antigen–antibody complexes activate complement and neutrophils, causing vasculitis, glomerulonephritis, arthritis and serum sickness.

7.3 Cytotoxic lymphocytes

CD8 cells and NK cells kill infected or abnormal cells through perforin/granzyme and death receptors. The resulting apoptosis may be accompanied by secondary inflammation.

7.4 Autoimmune stimulation

Persistent self-antigen drives chronic disease with acute flares. Examples include rheumatoid synovitis, inflammatory bowel disease, lupus and autoimmune hepatitis.

8. Crystal and metabolic stimuli

Crystal/metabolic signal	Response	Clinical example
Monosodium urate	NLRP3/IL-1β, neutrophils and intense synovitis.	Acute gout.
Calcium pyrophosphate	Crystal phagocytosis and neutrophilic inflammation.	Pseudogout.
Cholesterol crystals	Macrophage inflammasome activation and plaque inflammation.	Atherosclerotic plaque rupture.
Silica crystals	Lysosomal injury, inflammasome activation and fibrosis.	Silicosis.
Calcium oxalate	Tubular injury and crystal nephropathy.	Ethylene glycol toxicity or oxalosis.
Free fatty acids	Lipotoxicity and low-grade cytokine activation.	Obesity-associated inflammation and fatty liver.

9. Neoplastic and treatment-related stimuli

Tumours generate inflammation through necrosis, invasion, cytokines and immune recognition. Chemotherapy, radiotherapy, immunotherapy and targeted drugs can produce tumour-cell death or immune-related inflammation.

• Tumour lysis: rapid cell death releases potassium, phosphate and nucleic acids.
• Checkpoint-inhibitor toxicity: T-cell activation may cause colitis, hepatitis, pneumonitis, myocarditis or endocrinopathy.
• Radiation injury: DNA damage and endothelial inflammation can be acute or delayed.

10. Clinical method for identifying the stimulus

1. Timeline: minutes suggests anaphylaxis or toxin; hours suggests infection/trauma; weeks to months suggests autoimmune or foreign-body disease.
2. Exposure: drug, food, occupation, travel, trauma, surgery, transfusion or device.
3. Distribution: focal, dermatomal, joint-specific, pulmonary, gastrointestinal or systemic.
4. Fluid/tissue: pus, blood, clear blister fluid, fibrin, necrosis or granuloma.
5. Host factors: diabetes, neutropenia, immunosuppression, pregnancy, age and vascular disease.
6. Tests: cultures/PCR, imaging, toxicology, autoantibodies, biopsy and serial organ-function markers.

11. Emergency priorities

Stimulus suspected	First priorities
Anaphylaxis	IM adrenaline, airway/oxygen/circulation support, monitor for recurrence.
Severe infection	Resuscitation, antimicrobials when indicated, cultures without unsafe delay, source control.
Necrotising infection	Immediate surgical review, broad antibiotics, debridement and organ support.
Ischaemia/infarction	Restore perfusion where indicated, manage shock and reperfusion complications.
Toxin/overdose	Stop exposure, decontaminate when appropriate, antidote and poison-centre advice.
Crush/compartment syndrome	Perfusion assessment, urgent surgery, ECG/electrolyte/CK monitoring.
Acute autoimmune crisis	Protect the threatened organ, exclude infection, specialist immunotherapy.

12. Applied cases

Case 1: Sudden urticaria and wheeze

Minutes after a drug, the patient develops hives, bronchospasm and hypotension. The stimulus is IgE-mediated mast-cell activation. Give IM adrenaline immediately; do not wait for tryptase or antihistamine response.

Case 2: Acute limb pain after embolus

Absent pulses, pallor and paraesthesia indicate ischaemia. The stimulus is vascular occlusion, while downstream DAMPs will drive sterile inflammation and necrosis. Urgent vascular assessment and reperfusion planning are essential.

Case 3: Fever and shock after a wound

Rapid pain, bullae and systemic toxicity suggest necrotising infection. The trigger is microbial invasion with toxin and tissue necrosis. Antibiotics and immediate debridement must proceed in parallel.

13. Quick self-test

1. What is the difference between a PAMP and a DAMP?
   Answer: PAMPs come from microbes; DAMPs come from injured or dying host cells.
2. Name three sterile stimuli of acute inflammation.
   Answer: Infarction, trauma, burns, crystals, toxins, foreign bodies or radiation.
3. Which stimulus causes acute gout?
   Answer: Monosodium urate crystals.
4. What is the immediate emergency treatment for anaphylaxis?
   Answer: Intramuscular adrenaline.
5. Why can necrotic tissue inflame even without infection?
   Answer: DAMPs activate pattern-recognition receptors and inflammasomes.
6. What findings suggest necrotising infection rather than simple cellulitis?
   Answer: Pain out of proportion, rapid progression, bullae, crepitus, skin anaesthesia or systemic toxicity.
7. Why must an abscess often be drained?
   Answer: The purulent centre is poorly penetrated by immune cells and antibiotics.

14. Take-home summary

Selected references

• NCBI Bookshelf: Acute Inflammation
• NCBI Bookshelf: Inflammation and Repair
• NCBI Bookshelf: Anaphylaxis
• NCBI Bookshelf: Sepsis and Septic Shock

Educational note: This resource supports learning and clinical reasoning. Current local protocols, senior supervision and national guidelines take precedence in patient care.