Medical School · Year 2 · Pathology · includes a quiz and discussion video

Lecture 02: Acute Inflammation

Unit 2.11: Pathology

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the cardinal signs and functions of acute inflammation
2. Explain the vascular changes in acute inflammation
3. Describe leukocyte recruitment and activation
4. Explain the chemical mediators of inflammation
5. Describe the patterns and outcomes of acute inflammation
6. Explain the systemic effects of inflammation

Lecture Outline

I. Overview of Inflammation

Inflammation represents the body's protective response to tissue injury or infection, serving the fundamental purpose of eliminating the cause of cell injury, removing necrotic cells and debris, and initiating the process of tissue repair. This complex response involves coordinated vascular, cellular, and chemical components that work in concert to achieve these protective goals. The inflammatory response can be classified as acute when it develops rapidly over minutes to hours and resolves within days to weeks, or chronic when the process persists for weeks to years with ongoing tissue destruction and repair occurring simultaneously.

The cardinal signs of inflammation, first described by Celsus in ancient Rome and later expanded by Virchow, provide a clinical framework for recognizing inflammatory processes. Rubor (redness) results from vasodilation and increased blood flow to the affected area, bringing leukocytes and plasma proteins to the site of injury. Calor (heat) develops from increased blood flow and elevated metabolic activity within inflamed tissues. Tumor (swelling) arises from increased vascular permeability allowing plasma proteins and fluid to accumulate in the extravascular space, forming the characteristic inflammatory exudate.

Dolor (pain) occurs through multiple mechanisms including stimulation of nerve endings by inflammatory mediators such as bradykinin and prostaglandins, as well as increased tissue pressure from edema that stretches pain-sensitive structures. Functio laesa (loss of function), added by Virchow, represents the combined effect of pain, swelling, and tissue destruction that impairs normal organ function during inflammation. These clinical signs reflect the underlying vascular and cellular events that constitute the inflammatory response.

The stimuli that trigger inflammation encompass a wide range of injurious agents. Infections by bacteria, viruses, fungi, and parasites represent the most common cause of clinically significant inflammation. Tissue necrosis from any cause, including ischemia and trauma, releases damage-associated molecular patterns (DAMPs) that activate inflammatory pathways. Foreign bodies, including sutures and implants, provoke chronic inflammatory responses. Immune reactions in hypersensitivity diseases cause inflammation directed against either exogenous antigens or self-antigens in autoimmune conditions. Chemical irritants and toxins can directly injure tissues and trigger inflammatory responses.

<image>Panel A: Illustration of a limb showing all five cardinal signs of inflammation - redness, heat, swelling, pain, and loss of function - with labels indicating the underlying mechanisms for each sign. Panel B: Comparison diagram showing acute versus chronic inflammation with typical timeline, cell types involved, and outcomes. Panel C: Schematic showing various stimuli that trigger inflammation including pathogens, tissue necrosis, foreign bodies, and immune complexes converging on inflammatory pathways. Panel D: Flowchart depicting the protective purposes of inflammation including elimination of pathogens, removal of debris, and initiation of repair.</image>

II. Vascular Changes

The vascular changes in acute inflammation follow a characteristic sequence beginning with transient arteriolar vasoconstriction lasting only seconds, mediated by neurogenic mechanisms. This is rapidly followed by vasodilation, initially involving arterioles and then extending to the capillary bed, which is responsible for the increased blood flow (hyperemia) that causes erythema and warmth at the site of inflammation. The primary mediators of vasodilation include histamine released from mast cells, nitric oxide produced by endothelial cells, and prostaglandins generated through arachidonic acid metabolism.

Increased vascular permeability represents the hallmark of acute inflammation and is responsible for the protein-rich exudate that accumulates in inflamed tissues. The principal mechanism involves endothelial cell contraction in post-capillary venules, creating intercellular gaps through which plasma proteins can escape. This contraction is induced by histamine, bradykinin, and leukotrienes, and is a rapid, reversible response that occurs within minutes and typically resolves within 30 minutes. The process is termed the immediate transient response because of its quick onset and self-limited nature.

Other mechanisms of increased permeability operate in different contexts and timeframes. Direct endothelial injury from burns, microbial toxins, or chemical agents causes necrosis and detachment of endothelial cells, producing immediate and sustained leakage that may require vessel repair for resolution. Leukocyte-mediated injury occurs when activated neutrophils and other inflammatory cells release reactive oxygen species and proteolytic enzymes that damage adjacent endothelium. Increased transcytosis through the endothelial cell cytoplasm provides an additional pathway for protein extravasation.

The accumulation of protein-rich fluid in the extravascular space reduces the intravascular osmotic pressure while increasing extravascular oncotic pressure, promoting further fluid efflux and the development of edema. As fluid exits the vasculature, blood becomes more viscous with concentration of red blood cells, leading to stasis or slowing of blood flow. This stasis facilitates leukocyte margination, the process by which white blood cells accumulate along the vessel wall in preparation for emigration into tissues. The transition from transudate to protein-rich exudate distinguishes inflammatory edema from edema caused by hemodynamic disturbances such as heart failure.

<image>Panel A: Sequential diagram showing the vascular changes in inflammation from transient vasoconstriction through vasodilation to increased permeability and stasis. Panel B: Cross-section of a post-capillary venule showing endothelial cell contraction with gap formation and plasma protein leakage. Panel C: Comparison of normal endothelium, endothelial contraction, direct injury, and leukocyte-mediated damage with their respective mechanisms. Panel D: Illustration showing the progression from normal plasma flow to stasis with red cell concentration and leukocyte margination.</image>

III. Leukocyte Recruitment

Leukocyte recruitment to sites of inflammation occurs through a carefully orchestrated series of steps that enable circulating white blood cells to exit the vasculature and migrate to the site of injury. The process begins with margination, in which leukocytes move from the central axial stream of blood flow to the vessel periphery, a process facilitated by the stasis and hemoconcentration that develop during inflammation. Once at the vessel wall, leukocytes engage in rolling, a process of loose, transient adhesion mediated by selectins that allows cells to sample the endothelial surface for activation signals.

The selectin family of adhesion molecules plays the critical role in initiating leukocyte-endothelial interactions. E-selectin and P-selectin are expressed on activated endothelial cells, with P-selectin mobilized rapidly from preformed stores in Weibel-Palade bodies and E-selectin synthesized de novo following cytokine stimulation. L-selectin is constitutively expressed on leukocytes and binds to glycosylated ligands on endothelium. The selectin-mediated interactions are characterized by rapid on-off kinetics that produce the rolling movement, slowing leukocytes sufficiently for subsequent activation signals to be received.

Firm adhesion follows rolling and requires activation of integrin molecules on the leukocyte surface. Chemokines displayed on the endothelial surface trigger conformational changes in leukocyte integrins, converting them from a low-affinity to high-affinity state. The activated integrins, particularly LFA-1 and Mac-1, bind tightly to their endothelial counterligands ICAM-1 and VCAM-1, arresting leukocyte movement and anchoring the cell to the vessel wall. This adhesion strengthening is essential for the subsequent step of transmigration through the vessel wall.

Transmigration, also termed diapedesis, involves leukocyte movement through the endothelium, basement membrane, and pericytes into the extravascular tissue. The process occurs predominantly at intercellular junctions through interactions with adhesion molecules including PECAM-1 (CD31) expressed on both leukocytes and endothelial cell junctions. Once in the tissue, leukocytes migrate toward the site of injury along chemotactic gradients. Chemotaxis is directional movement guided by concentration gradients of chemoattractants including bacterial products such as N-formyl peptides, complement fragment C5a, leukotriene B4, and chemokines such as IL-8 (CXCL8) produced by macrophages and other tissue cells.

<image>Panel A: Step-by-step illustration of leukocyte recruitment showing margination, rolling, activation, firm adhesion, and transmigration through the vessel wall. Panel B: Molecular detail of selectin-mediated rolling showing E-selectin and P-selectin on endothelium interacting with sialyl-Lewis X on leukocytes. Panel C: Diagram of integrin activation showing chemokine-induced conformational change from bent inactive to extended active form with ICAM-1 binding. Panel D: Cross-section showing leukocyte transmigration at endothelial junctions with PECAM-1 interactions and subsequent chemotactic migration.</image>

IV. Leukocyte Activation

Leukocyte activation occurs when inflammatory cells recognize pathogens or tissue damage through specialized surface receptors that trigger intracellular signaling cascades. Pattern recognition receptors (PRRs), including the Toll-like receptor family, recognize conserved microbial structures called pathogen-associated molecular patterns (PAMPs) that are essential for microbial survival and thus difficult to mutate. TLR4 recognizes bacterial lipopolysaccharide, TLR2 recognizes bacterial lipoproteins and peptidoglycan, and TLR5 recognizes bacterial flagellin. These receptors also recognize damage-associated molecular patterns (DAMPs) released from necrotic cells.

Phagocytosis is the process by which leukocytes engulf and internalize particulate matter including microorganisms and debris. Recognition of targets is greatly enhanced by opsonization, the coating of particles with molecules such as IgG antibodies and complement fragment C3b that are recognized by specific receptors on phagocytes. Fc receptors bind the constant region of IgG, while complement receptors recognize C3b and its cleavage products. Upon binding, pseudopods extend around the particle, eventually fusing to form a membrane-bound phagosome.

The phagosome fuses with lysosomes to form the phagolysosome, where killing and degradation of ingested material occurs through both oxygen-dependent and oxygen-independent mechanisms. The oxidative burst, mediated by NADPH oxidase, generates superoxide anion that is converted to hydrogen peroxide and other reactive oxygen species. The myeloperoxidase-hydrogen peroxide-halide system generates hypochlorous acid (bleach), which is the most potent antimicrobial agent produced by neutrophils. Nitric oxide synthase produces reactive nitrogen species that contribute to microbial killing, particularly in macrophages activated by interferon-gamma.

Neutrophil extracellular traps (NETs) represent a recently discovered antimicrobial mechanism in which neutrophils release their nuclear contents to form extracellular fibrous structures. These NETs consist of chromatin decorated with antimicrobial proteins including elastase, myeloperoxidase, and defensins, creating a physical trap that immobilizes and kills bacteria and fungi. The process of NET formation, termed NETosis, typically results in neutrophil death. While NETs contribute to host defense, they may also contribute to pathology in autoimmune diseases such as systemic lupus erythematosus, where they provide a source of nuclear antigens and stimulate autoantibody production.

<image>Panel A: Diagram showing pattern recognition receptors on a macrophage surface recognizing various PAMPs including LPS, peptidoglycan, and flagellin with corresponding TLRs. Panel B: Sequential illustration of phagocytosis from opsonin recognition through pseudopod extension, phagosome formation, and phagolysosome fusion. Panel C: Schematic of the oxidative burst showing NADPH oxidase complex assembly, superoxide generation, and myeloperoxidase system producing hypochlorous acid. Panel D: Illustration of neutrophil extracellular trap formation showing nuclear decondensation, membrane rupture, and the released chromatin web trapping bacteria.</image>

V. Chemical Mediators - Cell-Derived

Histamine represents the principal vasoactive amine in human inflammation, stored in preformed granules within mast cells, basophils, and platelets. Upon activation by IgE-mediated degranulation, complement fragments C3a and C5a, or physical stimuli, histamine is rapidly released and acts on H1 receptors on endothelial cells to cause arteriolar dilation and increased venular permeability. The effects of histamine are rapid in onset but short-lived, lasting only 15-30 minutes, and are responsible for the immediate phase of increased vascular permeability. Serotonin, stored in platelet dense granules, has similar effects and is released during platelet aggregation.

Arachidonic acid metabolites, collectively termed eicosanoids, are generated from membrane phospholipids through the action of phospholipase A2 and subsequent metabolism through either the cyclooxygenase or lipoxygenase pathways. The cyclooxygenase pathway produces prostaglandins and thromboxane A2, while the lipoxygenase pathway generates leukotrienes and lipoxins. Prostaglandins, particularly PGE2 and PGI2, cause vasodilation and potentiate the edema-forming effects of histamine and bradykinin. PGE2 also sensitizes pain receptors and acts on the hypothalamus to induce fever, explaining why cyclooxygenase inhibitors such as aspirin and NSAIDs are effective analgesics and antipyretics.

Thromboxane A2, produced primarily by platelets, causes vasoconstriction and promotes platelet aggregation, playing important roles in hemostasis but potentially contributing to thrombotic complications of inflammation. The leukotrienes have distinct biological effects depending on their structure. Leukotriene B4 is a potent chemotactic agent for neutrophils and promotes their adhesion to endothelium. The cysteinyl leukotrienes (LTC4, LTD4, LTE4), previously known as slow-reacting substance of anaphylaxis, cause intense vasoconstriction, bronchospasm, and increased vascular permeability, making them central mediators in asthma pathophysiology.

Lipoxins represent a class of arachidonic acid metabolites with anti-inflammatory properties, providing endogenous stop signals that help resolve inflammation. Lipoxin A4 inhibits neutrophil chemotaxis and adhesion while promoting macrophage phagocytosis of apoptotic neutrophils. The balance between pro-inflammatory leukotrienes and anti-inflammatory lipoxins influences the intensity and duration of inflammatory responses. Therapeutic manipulation of eicosanoid pathways remains an important strategy, with cyclooxygenase inhibitors being among the most widely used medications worldwide and leukotriene receptor antagonists providing effective therapy for asthma.

<image>Panel A: Illustration of mast cell degranulation showing IgE cross-linking, granule release, and histamine binding to H1 receptors on endothelial cells causing gap formation. Panel B: Complete arachidonic acid metabolism diagram showing phospholipase A2 action, cyclooxygenase and lipoxygenase pathways with all major products and their functions. Panel C: Molecular structures and vascular effects of major prostaglandins including PGE2, PGI2, and thromboxane A2. Panel D: Diagram showing leukotriene synthesis with LTB4 neutrophil chemotaxis and cysteinyl leukotriene effects on bronchial smooth muscle.</image>

VI. Chemical Mediators - Cytokines and Others

The pro-inflammatory cytokines TNF-alpha and IL-1beta are the principal mediators responsible for initiating and amplifying the inflammatory response. Produced primarily by activated macrophages in response to pattern recognition receptor engagement, these cytokines act on endothelial cells to increase expression of adhesion molecules and enhance leukocyte recruitment. They induce fever through action on the hypothalamus, stimulate production of acute phase proteins by the liver, and cause metabolic wasting in chronic inflammatory states. In severe infections, excessive TNF-alpha release contributes to septic shock through effects on vascular tone and cardiac function.

Interleukin-6 shares many functions with TNF-alpha and IL-1, particularly in inducing the hepatic acute phase response, but also has some anti-inflammatory properties. Chemokines comprise a large family of small proteins that direct leukocyte migration through chemotaxis and regulate leukocyte activation. IL-8 (CXCL8) is the prototype chemokine, produced by macrophages and endothelial cells, and serves as the principal chemoattractant for neutrophils in acute inflammation. Different chemokines attract different leukocyte populations, providing specificity to the inflammatory cell infiltrate.

Anti-inflammatory cytokines counterbalance the pro-inflammatory response and promote resolution. IL-10, produced by regulatory T cells and alternatively activated macrophages, inhibits production of pro-inflammatory cytokines and reduces macrophage activation. TGF-beta has complex effects including suppression of lymphocyte proliferation and promotion of fibrosis during tissue repair. The balance between pro-inflammatory and anti-inflammatory cytokines determines the character and outcome of the inflammatory response.

Platelet-activating factor (PAF) is a phospholipid-derived mediator with potent pro-inflammatory effects that acts at extremely low concentrations, being 100 to 1000 times more potent than histamine in increasing vascular permeability. PAF causes platelet aggregation, bronchospasm, and vasodilation, and primes neutrophils and monocytes for enhanced responses to other stimuli. Nitric oxide, produced by endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS), mediates vasodilation and has antimicrobial activity in phagocytes. The short half-life of nitric oxide limits its effects to the immediate vicinity of its production, providing spatial specificity to its actions.

<image>Panel A: Diagram showing macrophage activation by PAMPs leading to TNF-alpha and IL-1 production with their systemic effects on hypothalamus, liver, and endothelium. Panel B: Chemokine gradient illustration showing IL-8 production by tissue macrophages creating a concentration gradient that directs neutrophil migration. Panel C: Balance scale showing pro-inflammatory cytokines (TNF, IL-1, IL-6) versus anti-inflammatory cytokines (IL-10, TGF-beta) and their effects on inflammation outcome. Panel D: Molecular pathway of nitric oxide synthesis in endothelial cells and macrophages with downstream effects on vasodilation and microbial killing.</image>

VII. Chemical Mediators - Plasma-Derived

The complement system comprises a cascade of plasma proteins that generate mediators of inflammation and contribute directly to microbial killing. Complement activation occurs through three pathways converging on the cleavage of C3. The classical pathway is initiated by antibody-antigen complexes binding C1, the alternative pathway is triggered by direct interaction of C3b with microbial surfaces, and the lectin pathway is activated by mannose-binding lectin recognizing carbohydrates on pathogens. All three pathways generate the critical enzyme C3 convertase that cleaves C3 into C3a and C3b.

The products of complement activation have distinct inflammatory functions. C3a and C5a, termed anaphylatoxins, bind to mast cells and trigger degranulation with histamine release, causing vasodilation and increased vascular permeability. C5a is also a potent chemotactic factor for neutrophils, monocytes, and eosinophils, directing leukocyte migration to sites of complement activation. C3b serves as an opsonin, coating microorganisms and enhancing their recognition and phagocytosis by cells bearing complement receptors. The membrane attack complex (C5b-9) inserts into bacterial membranes, creating pores that cause osmotic lysis, a mechanism particularly important for killing Neisseria species.

The kinin system generates bradykinin, a potent vasoactive peptide, through activation of the contact system. Factor XII (Hageman factor) is activated by contact with negatively charged surfaces, initiating a cascade that converts prekallikrein to kallikrein. Kallikrein cleaves high-molecular-weight kininogen to release bradykinin, which causes vasodilation, increased vascular permeability, and pain through action on sensory nerve endings. Bradykinin is rapidly inactivated by angiotensin-converting enzyme (ACE), explaining why ACE inhibitors can cause angioedema through bradykinin accumulation.

The coagulation system interacts extensively with inflammatory pathways, with Factor XII activation initiating both kinin generation and the intrinsic coagulation cascade. Thrombin, the terminal protease of the coagulation cascade, has direct pro-inflammatory effects including promotion of leukocyte adhesion and induction of chemokine production. Fibrin formed during coagulation provides a scaffold that supports leukocyte migration and localizes the inflammatory response. The fibrinolytic system, activated in parallel with coagulation, generates plasmin that cleaves fibrin and can also activate complement C3. These interconnections between hemostatic and inflammatory systems explain the close relationship between thrombosis and inflammation in many pathological conditions.

<image>Panel A: Complete complement cascade diagram showing classical, alternative, and lectin pathways converging at C3, with products C3a, C3b, C5a, and membrane attack complex and their functions. Panel B: Illustration of the kinin system showing Factor XII activation, kallikrein generation, and bradykinin release with effects on blood vessels and nerve endings. Panel C: Diagram of ACE inhibitor-induced angioedema mechanism with bradykinin accumulation. Panel D: Interconnection diagram showing relationships between coagulation, fibrinolytic, complement, and kinin systems with shared activation pathways.</image>

VIII. Patterns of Acute Inflammation

Serous inflammation is characterized by outpouring of a thin, protein-containing fluid derived from plasma or secreted by mesothelial cells lining body cavities. The exudate typically contains few inflammatory cells and forms when vascular permeability increases in the absence of marked tissue damage or bacterial infection. Common examples include the fluid-filled blisters that develop following burns or viral infections, and the accumulation of pleural or peritoneal effusions in inflammatory conditions. The serous exudate typically resolves completely without significant tissue damage once the underlying cause is addressed.

Fibrinous inflammation develops when vascular permeability is sufficiently increased to allow large proteins, including fibrinogen, to escape into the extravascular space. Fibrinogen is converted to fibrin, which deposits on tissue surfaces as a characteristic fibrinous exudate. This pattern is typical of inflammation affecting body cavity surfaces such as the pericardium and pleura. Fibrinous pericarditis produces a "bread and butter" appearance when the fibrin-coated pericardial surfaces rub against each other, heard clinically as a friction rub. If the fibrin is not completely removed by fibrinolysis and phagocytosis, it becomes organized by ingrowth of fibroblasts and blood vessels, leading to fibrous scarring and potential adhesions.

Suppurative or purulent inflammation is characterized by the production of large amounts of pus, which consists of dead neutrophils, liquefied necrotic tissue, and edema fluid. This pattern is typically caused by pyogenic bacteria, particularly staphylococci and streptococci, that attract neutrophils through chemotactic signals and cause tissue necrosis through toxin production. An abscess is a localized collection of pus within a tissue, consisting of a central purulent cavity surrounded by a wall of granulation tissue that walls off the infection but also impedes antibiotic penetration. Abscesses typically require drainage for resolution, as antibiotics alone cannot penetrate the avascular central cavity effectively.

Ulcerative inflammation describes a pattern in which an inflammatory process causes loss of the surface epithelium, creating a local defect in an epithelial surface. Ulcers develop on the skin, in the oral cavity, in the gastrointestinal tract, and in the genitourinary tract. The base of an ulcer typically shows acute inflammation with neutrophils, fibrinous exudate, and granulation tissue. Chronic ulcers develop when healing is impaired and may show prominent fibrosis and distortion of adjacent tissue. Examples include peptic ulcers of the stomach and duodenum, pressure ulcers in immobilized patients, and venous stasis ulcers in patients with chronic venous insufficiency.

<image>Panel A: Cross-section of skin showing serous inflammation with blister formation and clear fluid accumulation between epidermis and dermis. Panel B: Gross photograph and histology of fibrinous pericarditis showing shaggy fibrin deposits creating "bread and butter" appearance. Panel C: Cross-section of tissue abscess showing central pus collection with surrounding pyogenic membrane, granulation tissue, and fibrosis. Panel D: Comparison of acute and chronic ulcers showing surface epithelial loss, inflammatory infiltrate, granulation tissue, and varying degrees of fibrosis.</image>

IX. Outcomes of Acute Inflammation

Complete resolution represents the ideal outcome of acute inflammation, occurring when the injurious stimulus is eliminated, tissue damage is limited, and the affected tissue is capable of regeneration. The prerequisites for complete resolution include effective removal of the inciting agent, adequate drainage of exudate and debris, and intact tissue architecture with preserved stem cells. The resolution process involves neutralization and clearance of mediators, normalization of vascular permeability, cessation of leukocyte emigration, and phagocytic removal of debris by macrophages. Apoptosis of neutrophils and their subsequent phagocytosis by macrophages is particularly important, as this prevents release of toxic neutrophil contents that would perpetuate inflammation.

Healing by fibrosis occurs when tissue destruction is substantial and the affected tissue cannot regenerate, or when abundant fibrinous exudate is not adequately cleared and becomes organized. Fibroblasts migrate into the area of damage and deposit collagen, while new blood vessels form through angiogenesis. The resulting granulation tissue matures into scar tissue that replaces the original parenchyma with fibrous connective tissue. While scarring restores tissue integrity, it does not restore normal function, and excessive fibrosis can impair organ function. Examples include healing of myocardial infarction with collagenous scar formation and organization of fibrinous pleuritis into fibrous pleural adhesions.

Progression to chronic inflammation occurs when the acute inflammatory response fails to eliminate the inciting agent or when the agent persists despite the inflammatory response. Certain organisms, including mycobacteria and some fungi, resist killing by phagocytes and trigger chronic granulomatous responses. Autoimmune conditions lead to chronic inflammation because the self-antigens cannot be eliminated. Foreign materials that resist degradation provoke persistent chronic inflammatory responses. The transition to chronicity is marked by replacement of neutrophils with mononuclear cells, ongoing tissue destruction, and concurrent fibrosis.

Abscess formation represents a specific outcome in which purulent inflammation becomes walled off by surrounding tissue, creating a localized collection of pus. The abscess wall consists of granulation tissue and eventually fibrous tissue that contains the infection but also prevents effective antibiotic penetration to the central cavity. Untreated abscesses may spontaneously rupture and drain, spread locally into adjacent tissues, or disseminate through the bloodstream causing bacteremia and metastatic abscesses. Surgical drainage remains the definitive treatment for established abscesses, often combined with antibiotic therapy to address any residual or disseminated infection.

<image>Panel A: Timeline diagram showing complete resolution of inflammation with sequential mediator neutralization, vascular normalization, and debris clearance returning tissue to normal. Panel B: Comparison of tissue regeneration versus scar formation showing preserved versus destroyed tissue architecture and the role of stem cell survival. Panel C: Flowchart showing factors determining whether acute inflammation resolves, heals by fibrosis, or progresses to chronic inflammation. Panel D: Cross-section of abscess showing central pus, pyogenic membrane, granulation tissue wall, and diagram of treatment by incision and drainage.</image>

X. Systemic Effects of Inflammation

Fever represents one of the most common systemic manifestations of inflammation and infection, resulting from elevation of the hypothalamic set point for body temperature. The fever-inducing substances, termed pyrogens, include exogenous bacterial products such as lipopolysaccharide and endogenous mediators produced by host cells. The major endogenous pyrogens are the cytokines IL-1, IL-6, and TNF-alpha, which are released by macrophages in response to infection and act on the hypothalamus. These cytokines stimulate prostaglandin E2 synthesis in the hypothalamus, which resets the thermoregulatory center to a higher temperature, triggering responses such as vasoconstriction and shivering to raise body temperature to the new set point.

The acute phase response encompasses a constellation of systemic changes that occur within hours of the onset of infection or inflammation. Hepatocytes respond to IL-6 and other cytokines by altering their pattern of protein synthesis, increasing production of acute phase proteins while decreasing synthesis of other proteins. C-reactive protein (CRP) is the prototypical acute phase protein, increasing up to 1000-fold during inflammation and serving as an opsonin that enhances complement activation and phagocytosis. Fibrinogen levels increase, accelerating the erythrocyte sedimentation rate (ESR). Serum amyloid A protein serves as a precursor for AA amyloid in chronic inflammatory conditions.

Leukocytosis, an increase in circulating white blood cells, occurs in response to inflammatory cytokines and growth factors. The specific pattern of leukocyte elevation provides diagnostic clues to the underlying cause. Neutrophilia characterizes acute bacterial infections and reflects both release of bone marrow reserves and accelerated granulopoiesis. Lymphocytosis suggests viral infection or chronic infection. Eosinophilia indicates parasitic infection or allergic conditions. Monocytosis accompanies chronic infections and inflammatory conditions. Leukemoid reactions, with white blood cell counts exceeding 50,000 per microliter, may mimic leukemia but resolve when the underlying infection is treated.

Severe systemic inflammation, as occurs in sepsis, can produce life-threatening complications including hypotension, disseminated intravascular coagulation, and multi-organ failure. Septic shock results from excessive cytokine release, particularly TNF-alpha, causing vasodilation, increased vascular permeability, myocardial depression, and endothelial activation. The progression from compensated to decompensated shock involves loss of vascular tone, tissue hypoperfusion, and accumulation of metabolic acids. Disseminated intravascular coagulation consumes platelets and clotting factors, causing both thrombotic organ damage and bleeding diathesis. Multi-organ dysfunction syndrome represents the final common pathway of severe sepsis, with mortality increasing with each additional organ system involved.

<image>Panel A: Diagram of fever pathogenesis showing bacterial LPS triggering macrophage cytokine release, hypothalamic prostaglandin E2 synthesis, and thermoregulatory responses. Panel B: Illustration of liver producing acute phase proteins in response to IL-6 with list of proteins that increase (CRP, fibrinogen, SAA) and decrease (albumin, transferrin). Panel C: Chart showing different leukocytosis patterns associated with bacterial infection (neutrophilia), viral infection (lymphocytosis), parasites (eosinophilia), and chronic infection (monocytosis). Panel D: Flowchart of septic shock progression from infection through cytokine storm to vasodilation, DIC, and multi-organ failure.</image>

Summary

• Inflammation is the body's protective response to injury that eliminates the cause and initiates repair
• The five cardinal signs are rubor, calor, tumor, dolor, and functio laesa reflecting vascular and cellular events
• Vascular changes include vasodilation, increased permeability, and stasis facilitating leukocyte margination
• Leukocyte recruitment involves margination, rolling via selectins, firm adhesion via integrins, and transmigration
• Phagocytosis involves recognition, engulfment, and killing through the oxidative burst and other mechanisms
• Cell-derived mediators include histamine, prostaglandins, leukotrienes, and cytokines with distinct effects
• Plasma-derived mediators include complement fragments C3a and C5a, and bradykinin from the kinin system
• Patterns of acute inflammation include serous, fibrinous, suppurative, and ulcerative forms
• Outcomes include resolution, healing by fibrosis, chronic progression, and abscess formation
• Systemic effects include fever, acute phase protein response, and leukocytosis

Key Terms

This content is subject to the MIT License. © 2024–2026 Hibbert School of Medicine.

Read this lecture as Markdown