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.
Understanding Inflammation
Cells that live in tissues act as early warning sensors. Mast cells, macrophages, and dendritic cells detect microbes or damaged cell contents through pattern-recognition receptors. These receptors respond to patterns that are common in bacteria, viruses, or injured cells.
The first signals affect nearby blood vessels within seconds or minutes. Endothelial cells lining the vessels become more sticky, so white blood cells can slow down instead of passing straight through. Fluid and blood proteins leave the circulation at the affected site.
This fluid can dilute toxins and carry proteins that help clot, mark microbes, or destroy them. Swelling is therefore useful at first, though too much swelling can compress nerves or limit movement.
White blood cells do not enter tissue randomly. Their movement depends on matching surface molecules. Selectins create weak contacts that let a leukocyte roll along the vessel wall.
Chemokines then activate integrins on the leukocyte. Integrins make a strong attachment to endothelial proteins, allowing the cell to squeeze between endothelial cells. Once outside the vessel, the leukocyte follows a chemical concentration gradient toward the source of damage.
Neutrophils are built for fast action. They engulf microbes and release enzymes and reactive oxygen chemicals. These tools can kill bacteria, but they may harm healthy tissue if their release is poorly controlled.
Pus contains dead neutrophils, microbes, tissue fluid, and cell debris. It is evidence of a strong local response, not a substance that should be confused with the original infection.
Long-lasting inflammation changes the balance between protection and damage. Macrophages can take on different roles depending on the signals around them. Some release substances that destroy microbes and injured tissue.
Others encourage repair by stimulating new vessel growth and collagen production. Collagen strengthens a wound, yet excess collagen forms scar tissue called fibrosis. Fibrosis can reduce normal organ function in the liver, lungs, kidneys, or heart.
In some infections, the immune system builds a compact structure called a granuloma to wall off a hard-to-remove cause. Tuberculosis is a well-known example. Chronic inflammation can also be driven by immune mistakes, such as rheumatoid arthritis, where immune cells keep attacking joint tissues.
Resolution requires more than the fading of redness or pain. The body must stop producing danger signals, remove dead cells, and clear used-up inflammatory proteins. Macrophages perform much of this cleanup by engulfing dying neutrophils in a process called efferocytosis.
This cleanup changes macrophage behavior toward tissue repair. Specialized lipid mediators help switch off further cell recruitment without blocking necessary repair. Fever and raised blood markers such as C-reactive protein can show that inflammation is affecting the whole body.
Medicines act at different points in these pathways. Nonsteroidal anti-inflammatory drugs reduce prostaglandin production, while corticosteroids broadly reduce immune signaling.
When learning this topic, connect each mediator to a specific effect, then connect that effect to a symptom or clinical finding. This makes the pathway easier to reason through.
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.