INFLAMMATION PHYSIOLOGY: How the Body Detects Injury, Mobilises Defence and Initiates Repair
Inflammation is a coordinated physiological response to tissue injury, infection or harmful stimuli. It serves as one of the body’s primary defence mechanisms, designed to contain damage, eliminate threats and initiate the healing process. Far from being merely “swelling” or “redness,” inflammation represents a highly orchestrated interaction between immune cells, blood vessels and biochemical mediators. Understanding inflammation is essential for interpreting clinical presentations, from a simple cut to pneumonia or sepsis. It is also key to recognising when inflammation becomes excessive or dysregulated, contributing acute illness and chronic disease.
What You Need to Know
Inflammation is a fundamental physiological response that allows the body to detect injury or infection, mobilise immune defences, and initiate tissue repair. It can occur in acute or chronic forms. Acute inflammation develops rapidly and is classically associated with redness, heat, swelling, pain, and loss of function. These features arise from coordinated changes in local blood vessels and immune cell behaviour, particularly increased blood flow, altered capillary permeability, and migration of immune cells from the circulation into affected tissues.
Several tightly linked processes drive the early inflammatory response:
Vasodilation, which increases blood flow and accounts for redness and heat
Increased capillary permeability, allowing plasma proteins and immune cells to enter tissues and cause swelling
Leukocyte recruitment, particularly neutrophils and later monocytes, which migrate toward chemical signals released at the site of injury
Inflammation is initiated when tissue-resident cells detect pathogens or cellular damage through pattern recognition receptors. This detection triggers release of chemical mediators such as histamine, which rapidly increases vascular permeability, prostaglandins, which contribute to pain and fever, cytokines that coordinate immune activation, and complement fragments that amplify inflammation and attract immune cells. These mediators create a local environment that supports pathogen clearance and removal of damaged tissue. When this response resolves appropriately, inflammatory signals subside and tissue repair proceeds. Chronic inflammation develops when resolution fails, leading to persistent immune activation, ongoing tissue injury, and progressive fibrosis that can impair normal organ function.
Beyond the Basics
Recognition of Injury and Pathogens
Inflammation begins when tissue-resident immune cells detect danger. Macrophages, dendritic cells, and mast cells act as sentinels within tissues, continuously monitoring their environment through pattern recognition receptors. These receptors detect pathogen-associated molecular patterns from microbes and damage-associated molecular patterns released by injured or dying cells. Recognition of these signals triggers immediate cellular activation.
Activated resident cells release early inflammatory mediators such as tumour necrosis factor alpha, interleukin-1, and histamine. These mediators act locally on nearby blood vessels and stromal cells, priming the tissue for immune cell recruitment. Endothelial cells lining blood vessels are activated to express adhesion molecules, while surrounding cells begin producing chemokines that establish directional signals for circulating leukocytes.
Vascular Changes and Increased Permeability
One of the earliest visible features of inflammation is vasodilation, driven primarily by histamine, nitric oxide, and prostaglandins. Expansion of local blood vessels increases blood flow to the affected area, producing redness and warmth. This change improves delivery of oxygen, nutrients, and immune cells to damaged tissue.
At the same time, endothelial cells alter their junctions, becoming more permeable. Plasma proteins, antibodies, and complement components move out of the circulation and into surrounding tissues. This fluid shift causes swelling but also delivers critical immune mediators directly to the site of injury. Increased permeability allows immune responses to occur where they are needed, rather than remaining confined to the bloodstream.
Leukocyte Recruitment and Activation
Neutrophils are the first major immune cells recruited during acute inflammation. Guided by chemokines, they slow within blood vessels, adhere to activated endothelial cells, and migrate through vessel walls into tissues, a process known as diapedesis. Once in the tissue, neutrophils rapidly phagocytose microbes and cellular debris and release antimicrobial enzymes and reactive oxygen species.
Monocytes arrive later and differentiate into macrophages within tissues. These macrophages continue phagocytosis and act as immune regulators, producing cytokines that either sustain inflammation or promote resolution depending on the local environment. Macrophages also function as antigen-presenting cells, allowing inflammatory responses to transition into adaptive immunity when necessary.
Chemical Mediators and Amplification
A wide range of chemical mediators amplify and coordinate inflammatory responses. Histamine, bradykinin, prostaglandins, and leukotrienes contribute to vasodilation, increased permeability, and pain by acting on blood vessels and sensory nerves. These mediators account for many of the classic features of inflammation experienced clinically.
Cytokines such as interleukin-6 and tumour necrosis factor alpha extend inflammatory signalling beyond the local tissue. They induce systemic responses including fever and production of acute-phase proteins by the liver. Complement fragments further amplify inflammation by recruiting leukocytes and, through formation of the membrane attack complex, directly damaging microbial membranes. Together, these mediators create a rapidly escalating but coordinated defensive response.
Resolution and Healing
Inflammation is intended to be tightly controlled and time-limited. Once pathogens are cleared and tissue injury stabilises, anti-inflammatory signals begin to dominate. Cytokines such as interleukin-10 and transforming growth factor beta suppress further immune activation and promote resolution.
Macrophages undergo functional changes, shifting from inflammatory activity toward clearance and repair. They remove apoptotic cells, stimulate fibroblast activity, and promote angiogenesis, the formation of new blood vessels. Fibroblasts deposit collagen and extracellular matrix, forming granulation tissue that supports wound healing. When resolution proceeds effectively, tissue structure and function are restored. Failure of this process leads to chronic inflammation, characterised by persistent immune activation, progressive tissue injury, and fibrosis.
Clinical Connections
Inflammation accounts for many of the clinical signs seen in infection and tissue injury. Fever develops when inflammatory cytokines act on the hypothalamus to raise the body’s temperature set point. Redness and warmth occur due to increased blood flow from vasodilation, while swelling results from plasma proteins and fluid moving into tissues as vascular permeability increases. Pain and tenderness arise when inflammatory mediators stimulate sensory nerve endings and increase tissue pressure.
Patterns of inflammation seen in practice include:
Localised pain, swelling, and warmth at sites of infection or injury
Systemic fever and malaise during widespread cytokine release
Persistent immune activation causing tissue damage in chronic disease
Progressive structural change in organs affected by long-standing inflammation
When inflammatory responses are excessive or poorly controlled, tissue injury outweighs protection. In sepsis, widespread mediator release leads to vasodilation, capillary leak, hypotension, and organ dysfunction. In autoimmune disease and asthma, repeated or sustained inflammation alters normal tissue architecture and function. Chronic inflammation contributes to conditions such as atherosclerosis, rheumatoid arthritis, and inflammatory bowel disease by maintaining ongoing immune cell activation within affected tissues.
Laboratory markers such as C-reactive protein and erythrocyte sedimentation rate increase during systemic inflammation and are commonly used to support diagnosis, assess disease activity, and monitor response to treatment. Therapeutic approaches aim to interrupt specific points in the inflammatory cascade.
Non-steroidal anti-inflammatory drugs reduce prostaglandin production and relieve pain and swelling. Corticosteroids suppress cytokine release and immune cell recruitment. Biologic therapies block targeted inflammatory mediators involved in chronic disease. Knowledge of inflammatory physiology explains both the effectiveness of these treatments and the risks associated with excessive immune suppression.
Concept Check
What triggers the release of inflammatory mediators during the early stages of inflammation?
How do vascular changes contribute to the cardinal signs of inflammation?
What roles do neutrophils and macrophages play during acute inflammation?
Which chemical mediators drive pain, swelling and fever during inflammation?
Why is proper regulation of inflammation essential to preventing chronic disease?