Inflammation is a coordinated biological response of vascularised tissues to infection, tissue injury, harmful substances or other disturbances that activates immune, vascular and cellular mechanisms to eliminate threats and initiate repair. It is not itself a disease and is not inherently harmful. Appropriate inflammation is an essential protective process that helps contain pathogens, remove damaged material and create conditions for tissue recovery.
Classic signs of acute inflammation include redness, heat, swelling, pain and reduced function, although the exact manifestations depend on the tissue and cause. The process involves changes in local blood vessels, movement of immune cells into affected tissues, release of signalling molecules and activation of systems such as complement and coagulation. Inflammation can be acute and short lived or chronic and persistent, with substantially different consequences.
Acute inflammation begins when cells detect infection, tissue damage or other danger signals. Tissue resident cells such as macrophages, mast cells and epithelial cells can release or activate mediators including histamine, prostaglandins, leukotrienes, cytokines and chemokines. Blood vessels near the affected region can dilate, increasing local blood flow and contributing to redness and warmth. Vascular permeability can increase, allowing fluid and proteins to leave the circulation and enter tissues, producing swelling. Endothelial cells can express adhesion molecules that enable circulating leukocytes to slow down, attach to vessel walls and migrate through them. These changes create an environment in which immune cells and soluble defence factors can reach the affected tissue more effectively.
Neutrophils are often among the earliest circulating immune cells recruited during acute inflammation. They can migrate along chemical gradients toward sites of infection or damage, engulf microorganisms and debris, release antimicrobial substances and generate reactive molecules. Monocytes can enter tissues and differentiate into macrophage populations with roles in pathogen control, debris clearance and repair. Dendritic cells can capture antigens and help initiate adaptive immune responses. Complement proteins can promote inflammation and mark microbes for phagocytosis. Platelets and coagulation pathways can also interact with inflammatory processes, particularly during tissue injury. These mechanisms are interconnected, and their combined activity helps isolate threats, destroy harmful organisms and prepare damaged tissues for restoration.
Pain and swelling associated with inflammation arise from specific physiological mechanisms rather than simply from the presence of immune cells. Inflammatory mediators can sensitise sensory nerve endings, increasing pain responses. Increased vascular permeability causes fluid to accumulate in tissues, producing oedema. Fever can occur when inflammatory cytokines influence thermoregulatory mechanisms in the brain, raising the body's temperature set point. Fever and other systemic inflammatory responses can alter pathogen growth, immune activity and metabolism, although excessive responses can be harmful. Inflammation can also produce changes in appetite, energy use and behaviour through systemic signalling. These local and systemic effects illustrate that inflammation can extend beyond the immediately damaged tissue when signalling molecules circulate through the body.
Inflammation must eventually be controlled. Once the initiating threat has been reduced, anti inflammatory mediators, regulatory immune cells, changes in signalling and clearance of activated cells help resolve the response. Macrophages can remove dead cells and contribute to tissue repair. Some inflammatory cells undergo programmed cell death, while signalling pathways shift from recruitment and activation toward resolution and restoration. Successful resolution allows tissue structure and function to recover, although the outcome depends on the nature and severity of the injury. If the initiating stimulus persists or resolution fails, inflammation can become chronic. Chronic inflammation may involve macrophages, lymphocytes and other cells and can cause ongoing tissue remodelling and damage.
Chronic inflammation differs from acute inflammation in duration, cellular composition and consequences. It can occur when persistent infections, autoimmune processes, long term irritant exposure or other conditions maintain immune activation. Chronic inflammatory processes can contribute to tissue fibrosis, altered blood vessel formation, persistent cellular stress and progressive tissue injury. Chronic inflammation is associated with many diseases, but the precise mechanisms vary considerably among conditions. It should not be assumed that every long term disease is simply caused by excessive inflammation. Inflammation can be a cause, consequence or component of a broader pathological process. Research therefore examines the specific inflammatory pathways involved rather than treating inflammation as one uniform biological phenomenon.
Inflammation has important relationships with immunity and infection. It can be triggered by pathogen associated molecular patterns from microbes or damage associated molecular patterns released by injured cells. Innate immune receptors detect these signals and activate intracellular pathways leading to mediator production. Adaptive immune responses can also promote or modify inflammation through cytokines and other mechanisms. Inflammation can assist immune cells in reaching infected tissues but can also damage host tissues if the response is excessive, prolonged or misdirected. This balance is particularly important in autoimmune disease and severe infections, where immune mediated tissue injury can become a major component of pathology. Inflammation therefore represents both a protective mechanism and a potential source of disease when its regulation fails.
Anti inflammatory treatments work by interfering with particular components of inflammatory pathways. Some medicines reduce production of prostaglandins, while corticosteroids influence broad inflammatory gene regulation and immune activity. Other therapies target specific cytokines or immune pathways and are used for selected inflammatory or autoimmune conditions. However, suppressing inflammation can also reduce beneficial immune functions and increase susceptibility to some infections or impair aspects of healing. Consequently, inflammation should not be regarded as something that should always be eliminated. The appropriate objective in medicine is generally to control harmful or excessive inflammation while preserving necessary defence and repair. The modern understanding of inflammation therefore emphasises initiation, amplification, cellular recruitment, resolution, tissue repair and the specific biological context in which the response occurs.
