Causes of Inflammation: Infectious, Physical, Chemical, Immune, Metabolic and Traumatic Triggers

Learning outcomes

• Classify inflammatory causes into infectious, traumatic/physical, chemical, immune, foreign-body, metabolic and neoplastic categories.
• Explain how PAMPs, DAMPs, crystals, antibodies, complement and cytokines initiate inflammation.
• Use clinical clues to distinguish infection from sterile inflammation.
• Recognise common emergency triggers such as sepsis, anaphylaxis, burns, infarction, necrotising infection and toxic exposure.
• Choose appropriate confirmatory tests and identify when source control or antidote therapy is urgent.

1. Cause-to-response framework

Question	Why it matters
What was the trigger?	Determines antimicrobial, antitoxin, reperfusion, surgical or immunologic therapy.
Is it still present?	Persistent infection, foreign body or obstruction prevents resolution.
Is tissue necrotic?	Necrosis releases DAMPs and may require debridement or drainage.
Is the response sterile or infectious?	Inflammation alone cannot distinguish infection; cultures and imaging may be required.
Is inflammation local or systemic?	Systemic organ dysfunction requires resuscitation and urgent escalation.

2. Infectious causes

2.1 Bacteria

Bacterial PAMPs (lipopolysaccharide, peptidoglycan, lipoteichoic acid, flagellin and bacterial DNA) activate toll-like and NOD-like receptors. Exotoxins, pore-forming toxins, enzymes and direct invasion add tissue injury.

• Pyogenic bacteria: neutrophil-rich suppuration and abscess formation.
• Intracellular bacteria: macrophage and T-cell responses; granulomas or chronic inflammation.
• Toxin-mediated disease: systemic cytokine release, endothelial injury or neuromuscular dysfunction.
• Biofilms: persistent low-grade inflammation around catheters, prostheses and chronic wounds.

2.2 Viruses

Viral RNA or DNA activates endosomal and cytosolic sensors, inducing type I interferons and inflammatory cytokines. Tissue damage can result from viral cytolysis, apoptosis, pyroptosis or cytotoxic T-cell killing. Viral infection may therefore produce either acute inflammation or chronic immune-mediated injury.

2.3 Fungi

Fungal cell-wall components such as beta-glucans and mannans activate C-type lectin receptors. Neutrophils, macrophages and granulomas are important. Invasive fungi may cause vascular thrombosis, infarction and necrotising inflammation, especially in neutropenic or immunosuppressed patients.

2.4 Parasites

Helminths trigger type 2 inflammation with IL-4, IL-5, IL-13, IgE and eosinophils. Protozoa may cause macrophage, cytotoxic T-cell or neutrophil responses. Tissue migration, obstruction and chronic antigen release lead to fibrosis or granulomas.

2.5 Prions

Misfolded prion protein produces progressive neurodegeneration with minimal classic inflammation. This illustrates that tissue damage can be severe even when inflammatory cells are sparse.

2.6 Infection-associated emergencies

• Sepsis and septic shock.
• Meningitis/encephalitis with raised intracranial pressure.
• Necrotising fasciitis and gas gangrene.
• Severe pneumonia and acute respiratory distress.
• Invasive fungal disease with haemorrhage or infarction.

3. Tissue injury and necrosis

Dead or injured cells release DAMPs including ATP, HMGB1, mitochondrial DNA, uric acid and heat-shock proteins. Sterile inflammation develops after:

• Infarction: myocardial, cerebral, renal, splenic or bowel ischaemia.
• Crush injury: muscle necrosis, rhabdomyolysis and hyperkalaemia.
• Burns and frostbite: thermal injury and reperfusion.
• Pancreatitis: enzyme-mediated fat and pancreatic necrosis.
• Surgery and tissue handling: controlled injury followed by repair.

Sterile inflammation may coexist with infection later; an infarcted bowel or necrotic tumour can become secondarily infected.

4. Physical causes

Physical agent	Mechanism	Inflammatory examples	Emergency priority
Heat	Protein denaturation, dehydration, vascular damage.	Burn wound, heat stroke.	Airway, fluid/electrolyte support, cooling and burn referral.
Cold	Vasoconstriction, ice crystals, ischaemia and reperfusion.	Frostbite and hypothermia.	Rewarming when safe, perfusion and tissue assessment.
Mechanical trauma	Cell rupture, haemorrhage, oedema and DAMP release.	Crush injury, fractures and compartment syndrome.	Bleeding control, perfusion, fasciotomy assessment.
Radiation	DNA breaks and ROS.	Radiodermatitis, marrow injury, radiation pneumonitis.	Decontamination, specialist and supportive care.
Electricity/lightning	Thermal injury, arrhythmia, muscle necrosis.	Electrical burns and rhabdomyolysis.	ECG, trauma survey, electrolytes and renal monitoring.
Pressure/barotrauma	Mechanical disruption, gas bubbles or ischaemia.	Decompression sickness, ventilator injury.	Oxygen, pressure management and specialist referral.

5. Chemical and toxic causes

5.1 Direct irritants

Acids, alkalis, solvents, smoke and concentrated disinfectants damage membranes and proteins at the contact site. Corrosive ingestion may cause airway oedema, oesophageal necrosis, perforation and mediastinitis.

5.2 Systemic toxins and metabolic activation

Exposure	Mechanism/inflammatory injury	Clues
Paracetamol overdose	NAPQI depletes glutathione and causes centrilobular hepatic necrosis.	Delayed nausea, rising AST/ALT, INR and encephalopathy.
Cyanide	Blocks cytochrome oxidase and cellular oxygen use.	Severe lactic acidosis, cardiovascular collapse.
Carbon monoxide	Carboxyhaemoglobin and mitochondrial dysfunction.	Neurologic symptoms, headache, multiple exposed people.
Organophosphates	Acetylcholinesterase inhibition; secretions and bronchospasm.	Cholinergic toxidrome, fasciculations, bradycardia.
Heavy metals	Protein binding, enzyme inhibition and ROS.	Neuropathy, abdominal symptoms, renal injury.
Drug hypersensitivity	Immune-mediated tissue injury and eosinophilic inflammation.	Rash, fever, hepatitis, nephritis or anaphylaxis.

5.3 Occupational and environmental exposures

Silica, asbestos, coal dust, tobacco smoke, air pollution and pesticides cause persistent macrophage activation, oxidative stress, fibrosis or malignancy risk. A detailed occupational history is often more revealing than a routine laboratory test.

6. Immune-mediated causes

6.1 Autoimmunity

Loss of tolerance produces antibodies, immune complexes or T cells that attack host tissue. Examples include rheumatoid arthritis, systemic lupus erythematosus, autoimmune hepatitis, vasculitis, inflammatory bowel disease and autoimmune encephalitis.

6.2 Hypersensitivity reactions

Type	Main mechanism	Examples
I (immediate)	IgE, mast cells, histamine, leukotrienes.	Anaphylaxis, allergic asthma, urticaria.
II (antibody-mediated)	IgG/IgM against cell or matrix; complement, phagocytosis or receptor effects.	Autoimmune haemolysis, Goodpasture disease, Graves disease.
III (immune complex)	Deposited antigen–antibody complexes activate complement and neutrophils.	SLE, serum sickness, some vasculitides.
IV (T-cell mediated)	Th1/Th17 cytokines, cytotoxic T cells and macrophage activation.	Contact dermatitis, tuberculosis, type 1 diabetes.

6.3 Allograft and transplant injury

Antibodies and T cells recognise donor antigens, causing acute or chronic rejection. Vascular inflammation can produce thrombosis and organ infarction. Immunosuppression reduces rejection but increases infection and malignancy risk.

7. Foreign bodies and persistent material

Macrophages cannot easily digest splinters, sutures, talc, silicone, keratin, silica or retained surgical material. Frustrated phagocytosis produces giant cells, fibrosis and chronic inflammation. Remove the material when feasible; persistent “sterile abscess” or draining sinus warrants imaging and specialist evaluation.

8. Metabolic, crystal and vascular causes

8.1 Urate crystals

Monosodium urate crystals activate NLRP3 inflammasomes and IL-1β, causing acute gout. Neutrophils ingest crystals and release ROS, proteases and NETs, producing intense pain and swelling.

8.2 Calcium pyrophosphate

Calcium pyrophosphate crystals trigger acute or chronic synovitis (pseudogout), especially in older adults or metabolic disorders.

8.3 Cholesterol crystals

Cholesterol crystals in atherosclerotic plaques activate macrophages and inflammasomes. Plaque rupture exposes thrombogenic material and causes myocardial infarction or stroke.

8.4 Obesity and metabolic syndrome

Expanded adipose tissue becomes hypoxic and recruits macrophages. Adipokines, free fatty acids and mitochondrial stress produce low-grade inflammation that promotes insulin resistance, fatty liver and atherosclerosis.

8.5 Vascular stasis and thrombosis

Endothelial injury, turbulent flow and hypercoagulability produce thrombi. Ischaemic tissue releases DAMPs and generates sterile inflammation; reperfusion adds ROS and leukocyte injury.

9. Neoplasia-associated inflammation

Tumours can cause inflammation through necrosis, cytokine secretion, tissue invasion, immune recognition and tumour-associated macrophages. Chronic inflammation promotes angiogenesis, DNA damage, matrix remodelling and immune evasion. Conversely, anti-tumour immunity can injure adjacent tissue.

• Paraneoplastic fever and cachexia may be cytokine-driven.
• Obstruction causes secondary infection and tissue injury.
• Immunotherapy can produce immune-related adverse events in skin, colon, liver, lung, heart and endocrine organs.

10. Distinguishing infectious from sterile inflammation

Clue	Infection more likely	Sterile injury more likely
Exposure	Contact, wound, device, travel or immunosuppression.	Trauma, infarction, surgery, crystals or toxin.
Microbiology	Positive cultures/PCR or purulent drainage.	Repeated negative cultures with clear noninfectious trigger.
Imaging	Abscess, gas, cavitation or focal consolidation.	Infarct, haematoma, pancreatitis or contusion.
Response to antibiotics	Improves when source and organism are addressed.	No benefit; source control or anti-inflammatory treatment needed.
Important caveat	Early cultures can be negative.	Necrotic sterile tissue can become secondarily infected.

11. Diagnostic approach

1. History: onset, exposures, travel, trauma, drugs, immune disease, devices and occupation.
2. Examination: local pattern, pain severity, perfusion, airway, neurologic status and systemic toxicity.
3. Basic tests: FBC, CRP, renal/liver function, lactate, glucose, electrolytes, urinalysis and blood gas when severe.
4. Microbiology: cultures before antibiotics when this does not cause dangerous delay; tissue sampling is often superior to swabs.
5. Imaging: ultrasound, radiography, CT, MRI or angiography for abscess, obstruction, infarction, gas and deep extension.
6. Pathology: biopsy, special stains, immunofluorescence or molecular testing for immune, granulomatous and neoplastic causes.

12. Emergency management by cause

Cause suspected	Time-critical action
Sepsis/serious infection	Resuscitate, cultures without unsafe delay, empiric antimicrobials, source control and organ support.
Anaphylaxis	Intramuscular adrenaline, airway/oxygen/circulation support and observation for biphasic reactions.
Necrotising infection	Immediate surgical consultation, broad antibiotics, resuscitation and urgent debridement.
Ischaemia/infarction	Restore perfusion when indicated, treat shock and prevent reperfusion/metabolic complications.
Toxin/overdose	Stop exposure, ABCs, poison-centre advice and specific antidote if available.
Crush/rhabdomyolysis	Monitor potassium/ECG/CK/creatinine, manage hyperkalaemia and support renal perfusion.
Autoimmune crisis	Protect the affected organ, exclude infection and start specialist-directed immunotherapy.

13. Applied cases

Case 1: Acute gout

A painful red first metatarsophalangeal joint contains monosodium urate crystals. NLRP3-driven IL-1β recruits neutrophils. Confirm the cause when needed by synovial fluid microscopy and treat pain/inflammation while considering infection in any hot, swollen joint.

Case 2: Necrotising wound

Rapid pain, swelling, bullae and shock after a small wound indicate severe infection and tissue necrosis. Do not wait for a normal early radiograph. Begin resuscitation, broad antimicrobials and urgent surgical exploration.

Case 3: Drug hypersensitivity

Fever, rash, eosinophilia, hepatitis and kidney injury after a new medication suggest immune-mediated inflammation. Stop the offending drug, assess airway and organ function, exclude infection and seek specialist guidance.

14. Quick self-test

1. Name six broad causes of inflammation.
   Answer: Infection, tissue injury/necrosis, physical agents, chemicals/toxins, immune reactions, foreign bodies, metabolic/crystal disease and neoplasia (any six).
2. What are PAMPs and DAMPs?
   Answer: Pathogen-associated and damage-associated molecular patterns detected by pattern-recognition receptors.
3. Why does an abscess often require drainage?
   Answer: The pus-filled centre is poorly penetrated by immune cells and antibiotics, so source control is needed.
4. Which hypersensitivity type causes anaphylaxis?
   Answer: Type I IgE-mediated hypersensitivity.
5. How do urate crystals initiate gout?
   Answer: They activate the NLRP3 inflammasome and IL-1β, recruiting neutrophils.
6. Why can sterile inflammation later become infectious?
   Answer: Necrotic tissue, collections and damaged barriers provide a substrate for microbial invasion.
7. What is the key treatment difference between infection and sterile infarction?
   Answer: Infection requires antimicrobials/source control; infarction requires restoration of perfusion and management of the underlying vascular cause.
8. What red flags demand urgent surgical review?
   Answer: Pain out of proportion, rapid progression, bullae, crepitus, skin anaesthesia, systemic toxicity or suspected deep necrosis.

15. Take-home summary

Selected references

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

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