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Inflammation
Acute vs Chronic, Mediators, and Resolution
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Inflammation is the body's protective response to infection, tissue injury, and harmful stimuli. It helps eliminate the cause of damage, clear dead cells, and begin repair. Acute inflammation is usually rapid and short-lived, while chronic inflammation persists and can injure normal tissues. Understanding the difference matters because many common diseases, from appendicitis to atherosclerosis, involve inflammatory pathways.
Acute inflammation is driven by vascular changes and the movement of leukocytes, especially neutrophils, into injured tissue. Chemical mediators such as histamine, prostaglandins, leukotrienes, cytokines, and complement control vasodilation, permeability, pain, fever, and cell recruitment. Chronic inflammation involves ongoing macrophage and lymphocyte activation, tissue destruction, and attempts at healing through fibrosis and angiogenesis. Resolution is an active process that depends on removal of the trigger, clearance of mediators and cells, and production of pro-resolving signals such as lipoxins, resolvins, and anti-inflammatory cytokines.
Key Facts
- The 5 cardinal signs of inflammation are rubor, calor, tumor, dolor, and functio laesa.
- Acute inflammation typically lasts minutes to days and is dominated by neutrophils; chronic inflammation lasts weeks to years and is dominated by macrophages, lymphocytes, and plasma cells.
- Vasodilation increases blood flow, while increased vascular permeability allows protein-rich exudate to enter tissues.
- Leukocyte recruitment follows the sequence margination -> rolling -> adhesion -> diapedesis -> chemotaxis.
- Major mediator examples: histamine causes vasodilation and permeability; prostaglandins cause pain and fever; leukotrienes increase permeability and chemotaxis; IL-1 and TNF promote endothelial activation and fever.
- Resolution can lead to complete restoration, while persistent injury may lead to fibrosis, abscess formation, or chronic inflammatory disease.
Vocabulary
- Exudate
- Exudate is protein-rich fluid and cells that leave blood vessels during inflammation because vascular permeability increases.
- Chemotaxis
- Chemotaxis is the directed movement of leukocytes toward higher concentrations of chemical signals at a site of injury or infection.
- Neutrophil
- A neutrophil is a fast-responding white blood cell that is especially important in acute bacterial inflammation.
- Macrophage
- A macrophage is a phagocytic immune cell that removes debris, secretes cytokines, and plays a central role in chronic inflammation and repair.
- Fibrosis
- Fibrosis is the deposition of excess connective tissue, especially collagen, during chronic inflammation or healing.
Common Mistakes to Avoid
- Thinking acute inflammation is always mild and chronic inflammation is always severe, which is wrong because acute inflammation can be life-threatening and chronic inflammation can be low-grade but persistent.
- Confusing transudate with exudate, which is wrong because transudate is low in protein and usually caused by pressure changes, while exudate is protein-rich and linked to inflammation.
- Assuming neutrophils dominate all inflammatory responses, which is wrong because macrophages and lymphocytes are the main cells in chronic inflammation.
- Treating resolution as a passive fading of symptoms, which is wrong because resolution requires active mediator switching, clearance of cells and debris, and restoration of tissue balance.
Practice Questions
- 1 A patient develops a painful red skin lesion 12 hours after a bacterial cut. Name the dominant inflammatory cell at this stage and list two vascular changes that help produce redness and swelling.
- 2 A biopsy from inflamed tissue shows many macrophages, lymphocytes, angiogenesis, and fibrosis for 3 months after persistent infection. Classify the inflammation as acute or chronic and name two features that support your answer.
- 3 Explain why blocking prostaglandin synthesis can reduce pain and fever in inflammation, but may not completely stop leukocyte recruitment.