Acute Inflammation: Vascular Changes, Leukocyte Recruitment, Mediators, Exudates and Outcomes

Learning outcomes

• Describe the vascular and cellular phases of acute inflammation in sequence.
• Explain arteriolar dilation, endothelial contraction, direct injury, leukocyte-mediated injury and delayed permeability.
• Describe exudate, transudate, stasis, margination, rolling, adhesion, transmigration and chemotaxis.
• Explain phagocytosis, opsonisation, killing and the causes of leukocyte-mediated collateral injury.
• Identify major acute inflammatory mediators and their effects.
• Recognise morphologic patterns and outcomes: resolution, abscess, fibrosis and chronic inflammation.
• Apply the pathway to sepsis, pneumonia, burns, anaphylaxis and acute limb injury.

1. Sequence at a glance

Phase	Main event	Visible/clinical result
Recognition	Macrophages, mast cells, dendritic cells and endothelium detect PAMPs/DAMPs.	Mediator release begins.
Vascular change	Arteriolar dilation and increased permeability.	Redness, heat, swelling and exudate.
Haemodynamic stasis	Protein-rich fluid leaves vessels; flow slows.	Leukocyte margination.
Leukocyte recruitment	Rolling, adhesion, transmigration and chemotaxis.	Neutrophils arrive at the focus.
Elimination	Phagocytosis, degranulation, ROS and extracellular traps.	Microbial killing and debris removal.
Termination/repair	Mediator decay, efferocytosis, drainage and repair.	Resolution, scar, abscess or chronic inflammation.

2. Recognition and mediator release

Resident macrophages, mast cells and epithelial cells detect PAMPs and DAMPs through toll-like, NOD-like, C-type lectin and cytosolic nucleic-acid receptors. They release preformed histamine and newly synthesised lipid mediators and cytokines. Complement, kinins and coagulation pathways in plasma amplify the response.

3. Vascular changes

3.1 Transient vasoconstriction

A brief arteriolar constriction may occur immediately after injury. It is usually short and clinically unimportant compared with the later dilation.

3.2 Vasodilation

Histamine, nitric oxide and prostaglandins relax arteriolar smooth muscle. Blood flow rises, producing redness and heat. Capillary beds open, increasing hydrostatic pressure and favouring fluid movement into tissue.

3.3 Increased permeability and exudation

Mechanism	Onset/duration	Typical triggers
Endothelial contraction	Immediate, short-lived.	Histamine, bradykinin, leukotrienes.
Junctional retraction	Delayed, prolonged.	TNF and IL-1; cytoskeletal rearrangement.
Direct endothelial injury	Immediate or delayed, long-lasting.	Severe burns, toxins, necrotising infection.
Leukocyte-mediated injury	Delayed after leukocyte adhesion.	Venules and pulmonary capillaries in severe inflammation.
Increased transcytosis	Rapid, mediator-dependent.	VEGF and selected cytokines.
New-vessel leak	Persistent until maturation.	Angiogenesis during repair and chronic inflammation.

3.4 Exudate versus transudate

An exudate is protein-rich inflammatory fluid containing leukocytes and debris. A transudate is protein-poor fluid caused by hydrostatic or oncotic imbalance without primary endothelial injury. The distinction helps separate inflammation from heart failure, cirrhosis and nephrotic oedema.

4. Haemodynamic stasis and margination

Loss of plasma concentrates red cells and increases blood viscosity. Slower flow pushes leukocytes from the central axial stream toward the endothelial surface (margination). Activated endothelium expresses selectins, integrin ligands and chemokines that prepare leukocytes for arrest.

5. Leukocyte recruitment

5.1 Rolling

Endothelial E-selectin and P-selectin bind transient carbohydrate ligands on leukocytes. The bonds form and break rapidly, so cells roll along the vessel wall. Histamine and thrombin rapidly mobilise P-selectin from endothelial stores; TNF and IL-1 induce E-selectin over hours.

5.2 Firm adhesion

Endothelial chemokines activate leukocyte integrins (LFA-1, Mac-1 and VLA-4), increasing their affinity. Integrins bind ICAM-1 and VCAM-1 on endothelium. Defects in integrins or their ligands cause recurrent infections and poor wound healing.

5.3 Transmigration (diapedesis)

Leukocytes squeeze between endothelial cells in post-capillary venules. PECAM-1 (CD31) and junctional molecules help the cell cross the basement membrane. Matrix metalloproteinases and proteases facilitate passage.

5.4 Chemotaxis

Leukocytes move along chemical gradients toward the highest concentration of attractant.

Chemoattractant	Source/importance
Bacterial peptides (N-formyl-methionyl peptides)	Directly signal microbial products.
C5a	Complement-derived chemotactic and activating fragment.
LTB4	Potent neutrophil chemoattractant and activator.
Chemokines (e.g. CXCL8/IL-8)	Endothelial and leukocyte-derived recruitment signals.
Platelet-activating factor	Promotes adhesion, chemotaxis and degranulation.

6. Phagocytosis and killing

6.1 Recognition and attachment

Phagocytes bind microbes directly through pattern-recognition receptors or through opsonins. Important opsonins include IgG, C3b, collectins and pentraxins. Opsonisation greatly increases the speed and efficiency of engulfment.

6.2 Engulfment

Pseudopods surround the particle and fuse to form a phagosome. The phagosome fuses with lysosomes to create a phagolysosome. Actin rearrangement, small GTPases and cytoskeletal motors are required.

6.3 Killing and degradation

• Respiratory burst: NADPH oxidase generates superoxide and hydrogen peroxide.
• Myeloperoxidase: H₂O₂ plus chloride forms hypochlorous acid.
• Reactive nitrogen species: inducible nitric oxide synthase produces NO and peroxynitrite.
• Non-oxidative mechanisms: defensins, lysozyme, proteases, lactoferrin and acidic lysosomes.
• Macrophage processing: antigens may be presented to T cells after digestion.

6.4 Leukocyte-mediated tissue injury

Neutrophils release enzymes and ROS into extracellular space when they encounter immune complexes, crystals, large particles or frustrated phagocytosis. Collateral damage causes emphysema, vasculitis, acute lung injury, pancreatitis and reperfusion injury.

7. Major acute inflammatory mediators

Mediator	Main effects
Histamine	Immediate vasodilation and venular permeability; bronchoconstriction in allergy.
Prostaglandins	Vasodilation, pain, fever and platelet modulation.
Leukotrienes	Bronchoconstriction, permeability and chemotaxis.
Bradykinin	Pain, permeability, vasodilation and smooth-muscle contraction.
Complement C3a/C5a	Mast-cell activation, chemotaxis, opsonisation and membrane attack complex.
TNF/IL-1	Endothelial activation, fever, leukocyte recruitment and systemic effects.
IL-6	Hepatic acute-phase response and fever.
PAF	Platelet activation, permeability, bronchoconstriction and leukocyte adhesion.
NO	Vasodilation and microbial killing; excessive levels contribute to shock.
Chemokines	Selective, directed leukocyte recruitment.

8. Morphologic patterns of acute inflammation

Pattern	Main content	Examples
Serous	Watery, relatively cell-poor fluid.	Burn blister, viral serositis.
Fibrinous	Fibrin-rich exudate.	Fibrinous pericarditis.
Suppurative	Neutrophils, bacteria and liquefied debris.	Abscess, bacterial pneumonia.
Catarrhal	Mucus-rich mucosal response.	Viral rhinitis and bronchitis.
Haemorrhagic	Red cells mixed with exudate.	Severe vascular injury, necrotising infection.
Ulcerative	Loss of epithelium with inflamed base.	Peptic ulcer or pressure sore.

9. Outcomes of acute inflammation

9.1 Resolution

Resolution is likely when injury is limited, the tissue can regenerate and the cause is removed. Neutrophils undergo apoptosis, macrophages clear debris, lymphatics drain oedema and normal architecture returns.

9.2 Healing by fibrosis

Large defects, tissue destruction, fibrin persistence or non-regenerating organs heal with collagen scar. Scar restores strength but can impair movement or organ function.

9.3 Abscess formation

Pyogenic bacteria and neutrophils produce a walled-off collection. Drainage may be necessary because the centre contains necrotic debris and has poor antibiotic penetration.

9.4 Progression to chronic inflammation

Persistent organisms, foreign bodies, toxins or autoimmune stimuli keep macrophages and lymphocytes activated. Ongoing injury and repair produce fibrosis and organ remodelling.

10. Systemic acute inflammation

• Fever: IL-1/TNF induce PGE₂ in the hypothalamus.
• Leukocytosis: neutrophilia, lymphocytosis or eosinophilia depending on cause; severe sepsis can cause leukopenia.
• Acute-phase proteins: CRP, fibrinogen, serum amyloid A and hepcidin rise.
• Tachycardia and metabolic demand: catecholamines, fever and cytokines increase cardiac workload.
• Sepsis/shock: widespread endothelial activation, vasodilation, leak and microthrombi cause organ dysfunction.

11. Clinical application

11.1 Pneumonia

Alveolar macrophages detect organisms and recruit neutrophils. Exudate fills alveoli, causing crackles, hypoxia and consolidation. Severe inflammation can produce ARDS; assess oxygenation, work of breathing, sepsis and need for escalation.

11.2 Anaphylaxis

Mast-cell histamine and lipid mediators cause rapid permeability, bronchospasm and vasodilation. Intramuscular adrenaline is time-critical; antihistamines and corticosteroids are adjuncts.

11.3 Burns and trauma

Direct endothelial injury and DAMPs create local and systemic oedema. Airway assessment, fluid/temperature management, analgesia, perfusion and infection prevention are essential.

11.4 Acute limb inflammation/compartment syndrome

Swelling within a closed fascial space raises pressure, compresses microvessels and converts reversible muscle injury into necrosis. Severe pain, pain on passive stretch, paraesthesia or weakness need urgent surgical review.

12. Quick self-test

1. Which vessels are most important for leukocyte transmigration?
   Answer: Post-capillary venules.
2. What causes leukocyte rolling?
   Answer: Selectin-mediated transient adhesion.
3. Which molecules mediate firm adhesion?
   Answer: Activated leukocyte integrins binding endothelial ICAM-1/VCAM-1.
4. What is the key neutrophil enzyme system for hypochlorous acid?
   Answer: Myeloperoxidase using hydrogen peroxide and chloride.
5. What distinguishes exudate from transudate?
   Answer: Exudate is protein-rich due to inflammatory permeability; transudate is protein-poor from hydrostatic/oncotic imbalance.
6. Name two chemoattractants.
   Answer: C5a, LTB4, CXCL8/IL-8 or bacterial N-formyl peptides.
7. What are the four major outcomes of acute inflammation?
   Answer: Resolution, fibrosis/scar, abscess formation or progression to chronic inflammation.
8. Why is severe acute inflammation dangerous?
   Answer: Oedema, leukocyte injury, vasodilation, microthrombi and cytokines can cause organ failure and shock.

13. Take-home summary

Selected references

• NCBI Bookshelf: Acute Inflammation
• NCBI Bookshelf: Inflammation and Repair
• 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.