Definition

Acute inflammation is a rapid and short-duration inflammatory response of vascularized tissue to injury or infection. It is characterized mainly by exudation of fluid and plasma proteins and accumulation of neutrophils at the site of injury.

How you know it's happening (5 classic signs):

• 🔴 Redness
• 🔥 Heat
• 🫧 Swelling
• 😣 Pain
• ⚙️ Loss of function (the part doesn't work normally)
  
  
  
  

  Morphological Patterns of Acute Inflammation

  Based on the severity of the reaction, causative agent, and the tissue/organ involved, acute inflammation can present in several distinct morphologic patterns:

  1. Serous Inflammation

  • Characterized by outpouring of thin, protein-poor fluid (derived from blood plasma or secretions of mesothelial cells).
  • Example: Skin blister from a burn or viral infection; fluid in pleural, peritoneal, or pericardial cavities (effusion).

  2. Fibrinous Inflammation

  • Occurs when the vascular leak is severe enough to allow large molecules like fibrinogen to pass through and form fibrin.
  • Typically develops on lining surfaces such as pericardium and pleura.
  • Example: Fibrinous pericarditis ("bread and butter" appearance).
  • May resolve completely (resolution) or organize into scar tissue if not removed (organization).

  3. Suppurative (Purulent) Inflammation

  • Characterized by production of pus — an exudate composed of neutrophils, necrotic cells, and edema fluid.
  • Usually caused by pyogenic bacteria (e.g., Staphylococcus).
  • Abscess: A localized collection of pus within a tissue, organ, or confined space, surrounded by inflamed tissue.
  • Example: Acute appendicitis, boils, abscesses.

  4. Ulcer

  • A local defect (excavation) of the surface of an organ or tissue produced by sloughing (shedding) of inflamed necrotic tissue.
  • Occurs when tissue necrosis and resulting inflammation exist on or near a surface.
  • Example: Peptic ulcer, ulcers of the mouth or GI tract, skin ulcers in the lower extremities in the elderly.

  5. Pseudomembranous Inflammation

  • A special pattern occurring on mucosal surfaces (e.g., GI, respiratory tract) where a necrotizing inflammatory exudate combines with a superficial layer of fibrin and necrotic epithelial cells, forming a membrane-like layer ("pseudomembrane") over the mucosa.
  • Example: Clostridioides difficile colitis (pseudomembranous colitis), diphtheria of the throat.

  6. Catarrhal Inflammation

  • Inflammation of a mucous membrane, characterized by hypersecretion of mucus.
  • Example: Common cold (catarrhal rhinitis).

  7. Hemorrhagic Inflammation

  • Occurs when there is severe vascular damage leading to significant leakage of red blood cells along with the inflammatory exudate.
  • Example: Severe viral hemorrhagic fevers, certain necrotizing infections.

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  Quick summary table:

  Pattern	Key Feature	Example
  Serous	Thin, protein-poor fluid	Skin blister
  Fibrinous	Fibrin-rich exudate	Fibrinous pericarditis
  Suppurative/Purulent	Pus formation	Abscess, appendicitis
  Ulcer	Surface tissue defect	Peptic ulcer
  Pseudomembranous	Necrotic mucosal membrane	C. difficile colitis
  Catarrhal	Excess mucus secretion	Common cold
  Hemorrhagic	RBC-rich exudate	Hemorrhagic fever