Acute inflammation is a rapid, initial protective response to injury involving vascular permeability changes, exudation, and cellular recruitment. The cardinal signs—rubor, tumor, calor, dolor, functio laesa—reflect vasodilation, edema, neutrophil infiltration, and mediator release.
Study the temporal sequence: immediate vascular response, acute exudation (neutrophil predominance), and resolution or progression. Use histologic images to identify key morphologic features in different tissues.
Acute inflammation is not synonymous with infection—sterile trauma triggers the same response. Resolution of acute inflammation does not always restore normal function; chronicity and fibrosis can result.
You already know from your study of the innate immune response that the body has rapid, non-specific defenses that react within minutes to hours. Acute inflammation is the tissue-level manifestation of that response — the coordinated change in blood vessels and leukocyte behavior that you can observe clinically as redness, swelling, heat, and pain.
The response unfolds in a defined temporal sequence. First, tissue injury releases signals — either pathogen-associated molecular patterns (PAMPs) from microbes or danger-associated molecular patterns (DAMPs) from damaged host cells. These signals activate resident mast cells and macrophages, which release histamine and prostaglandins. Histamine causes immediate, reversible dilation of arterioles and increased permeability of post-capillary venules. The result: more blood reaches the tissue (rubor, calor) and protein-rich plasma leaks into the interstitium (tumor). This exudate carries antibodies and complement into the site.
Within hours, cytokines and chemokines — especially IL-1, TNF-α, and IL-8 — create chemical gradients that recruit circulating neutrophils. Neutrophils roll along the dilated endothelium (selectin-mediated), adhere firmly (integrin-mediated), and crawl through the vessel wall (transmigration). They are the dominant cell type in acute inflammation and serve as the first-wave effectors: they phagocytose debris and pathogens, release antimicrobial enzymes and reactive oxygen species, and form neutrophil extracellular traps (NETs). After 24-48 hours, monocytes arrive and differentiate into macrophages that coordinate resolution or progression.
A key conceptual point: acute inflammation is protective, not pathological in origin. The same cascade that responds to a bacterial abscess also responds to a sprained ankle or a myocardial infarction. In the absence of infection, the response is called sterile inflammation — triggered by the same innate receptors detecting DAMPs instead of PAMPs. This is why anti-inflammatory agents (NSAIDs, corticosteroids) can reduce the symptoms of both infection and injury, though at the cost of impairing the protective response.
Resolution is not guaranteed. If the injurious stimulus persists — or if the acute response is unable to eliminate it — the process can transition to chronic inflammation, characterized by lymphocyte and macrophage infiltration, tissue destruction, and fibrosis. Understanding this transition is essential for making sense of conditions like rheumatoid arthritis, tuberculosis, and atherosclerosis, all of which involve dysregulated or persistent inflammatory states.