Inflammation and Wound Healing

Core Idea

Inflammation is the coordinated vascular and cellular response to tissue injury or infection, characterized by the cardinal signs: redness (rubor, from vasodilation), warmth (calor, from increased blood flow), swelling (tumor, from increased vascular permeability and edema), pain (dolor, from prostaglandins and bradykinin sensitizing nociceptors), and loss of function. Tissue injury releases mediators (histamine, prostaglandins, cytokines) that increase local blood flow and allow plasma proteins and leukocytes to extravasate into tissue. Wound healing proceeds through overlapping phases: hemostasis (platelet plug and fibrin clot, minutes); inflammation (neutrophil then macrophage influx, days); proliferation (fibroblast collagen synthesis, angiogenesis, re-epithelialization, days to weeks); remodeling (collagen reorganization into mature scar, weeks to months). Failure to resolve inflammation leads to chronic inflammatory states.

How It's Best Learned

Assign a dominant cell type to each healing phase: hemostasis (platelets) → inflammation (neutrophils first 24–48 h, then macrophages) → proliferation (fibroblasts, endothelial cells, keratinocytes) → remodeling (myofibroblasts and matrix metalloproteinases). Explain why NSAIDs (prostaglandin synthesis inhibitors) reduce fever and pain but may impair healing if used excessively in the proliferative phase.

Common Misconceptions

• Inflammation is a protective and necessary response — suppressing it too aggressively (high-dose steroids) impairs pathogen clearance and delays healing.
• Scar tissue is structurally different from the original dermis: it contains parallel rather than basket-weave collagen, lacks hair follicles and sweat glands, and has altered mechanical properties.
• Neutrophils are short-lived (hours to days at the injury site) and must be replaced by longer-lived macrophages to sustain the response.

Explainer

When tissue is damaged — a cut, a burn, an invading bacterium — the innate immune response you have already studied provides the initial defense. Inflammation is the orchestrated local version of that response, and understanding it means understanding a carefully timed sequence of cellular events, each phase setting the stage for the next. The process begins within seconds of injury. Damaged cells release chemical alarms — histamine from mast cells, prostaglandins from injured membranes, and cytokines like interleukin-1 and tumor necrosis factor. These mediators cause local arterioles to dilate and capillary walls to become more permeable. The result is the classic signs you can observe directly: redness and warmth from increased blood flow, swelling from plasma leaking into the interstitial space, and pain from prostaglandins and bradykinin sensitizing local nerve endings.

The increased blood flow and vascular permeability serve a purpose: they deliver immune cells to the injury site. Neutrophils arrive first, typically within hours, squeezing through capillary walls in a process called diapedesis and following chemical gradients to the damaged area. Neutrophils are aggressive but short-lived — they phagocytose bacteria and debris, release antimicrobial enzymes, and die within a day or two, forming much of what we recognize as pus. Within 24–48 hours, macrophages take over. These longer-lived cells not only continue phagocytosis but also release growth factors that signal the transition from destruction to repair. Macrophages are the critical bridge between the inflammatory and proliferative phases — without them, wounds stall.

The proliferative phase begins several days after injury and is dominated by rebuilding. Fibroblasts migrate into the wound bed, laying down collagen to form granulation tissue — a temporary scaffold that fills the wound. Simultaneously, new blood vessels sprout from existing ones (angiogenesis) to supply the growing tissue, and epithelial cells at the wound edges divide and migrate across the surface in a process called re-epithelialization. The wound gradually contracts as specialized myofibroblasts pull the edges together. This phase can last days to weeks depending on wound size.

The final remodeling phase extends for months or even years. The initially disorganized collagen deposited during proliferation is gradually broken down by matrix metalloproteinases and replaced with more organized, cross-linked collagen fibers. Despite this remodeling, scar tissue never fully recapitulates the original architecture: it contains parallel collagen bundles rather than the basket-weave pattern of normal dermis, lacks hair follicles and sweat glands, and typically reaches only about 80% of the tensile strength of uninjured skin. This is why understanding inflammation is not just about fighting infection — it is about understanding the body's entire tissue repair program, from the initial alarm through the final scar.