🔥 Acute inflammation is a rapid, early defensive response of the body to harmful stimuli such as pathogens, damaged cells, or irritants. It is characterised by the classical signs of ➡️ heat, redness, swelling, pain, and loss of function. Its main purposes are to eliminate the cause of injury, remove necrotic debris, and initiate tissue repair.

🔄 Stages of Acute Inflammation

1. Initiation 🛎️ :
- Recognition of harmful stimuli via pattern recognition receptors (PRRs) on immune cells.
- Release of inflammatory mediators such as histamine, prostaglandins, cytokines.
2. Vascular Changes 💉 :
- Vasodilation: Increased blood flow → redness & heat.
- Increased vascular permeability: Plasma proteins & fluid leak into tissues → oedema (swelling).
3. Cellular Events 🧑‍⚕️🦠 :
- Leukocyte recruitment: Margination → rolling → adhesion → transmigration (diapedesis).
- Phagocytosis: Neutrophils & macrophages engulf debris → fuse phagosome with lysosome → destruction by ROS & enzymes.

🧪 Chemical Mediators of Inflammation

Vasoactive Amines ⚡ :
- Histamine: Mast cells, basophils, platelets → vasodilation & permeability.
- Serotonin: Platelets & enterochromaffin cells → similar vascular effects.
Plasma Protein Systems 🩸 :
- Complement (C3a, C5a): Anaphylatoxins → permeability & chemotaxis. C3b acts as an opsonin.
- Coagulation: Fibrin meshwork forms a clot scaffold.
- Kinin system: Bradykinin → vasodilation, pain, permeability.
Arachidonic Acid Metabolites 🌀 :
- Prostaglandins (COX): Pain, vasodilation, fever.
- Leukotrienes (LOX): Chemotaxis, permeability, bronchospasm.
Cytokines & Chemokines 📢 :
- TNF, IL-1: Fever, adhesion molecules, systemic acute phase response.
- Chemokines: Recruit leukocytes to the site.
ROS & Nitric Oxide ⚔️ :
- Microbicidal and signalling roles.
- Excess = tissue injury (ARDS, reperfusion injury).

🧭 Outcomes of Acute Inflammation

✅ Resolution: Full clearance & return to normal tissue.
💥 Abscess: Localised pus collection when clearance fails.
♻️ Chronic inflammation: If stimulus persists → fibrosis & immune activation.
🧱 Fibrosis/scarring: When tissue repair is incomplete.

🩺 Clinical Signs & Symptoms ("Cardinal Signs")

🔴 Redness (Rubor): Vasodilation, hyperaemia.
🔥 Heat (Calor): Increased blood flow & metabolic activity.
💧 Swelling (Tumour): Oedema from plasma leakage.
😣 Pain (Dolor): Mediated by bradykinin, prostaglandins, pressure effects.
⚠️ Loss of Function (Functio Laesa): Pain & swelling limiting use.

📚 Clinical Relevance

Fever in systemic inflammation is driven by IL-1 & TNF acting on the hypothalamus.
Raised CRP & ESR are acute phase reactants produced by the liver.
Uncontrolled acute inflammation can → sepsis, SIRS, or chronic inflammation (e.g. Crohn’s, RA).

📝 Summary

Acute inflammation is an essential protective response, aimed at eliminating harmful agents and initiating repair. It integrates vascular changes, leukocyte activity, and chemical mediators. ⚖️ While vital, if unchecked it can cause collateral tissue injury or evolve into chronic disease.

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Acute Inflammation