2.7.4 Inflammation

Introduction

Inflammation is the body’s coordinated response to tissue injury or infection. It involves blood vessels, immune cells, and chemical mediators and aims to eliminate the initial cause of cell injury, remove necrotic cells and tissues, and begin repair. For USMLE purposes, you must distinguish acute from chronic inflammation, understand the sequence of events in each, recognize major mediators, and know typical clinical and morphologic patterns.

This chapter focuses on what is specific to inflammation as a process, not on individual diseases that involve inflammation, which are covered in systemic pathology and other organ system chapters.

Acute versus Chronic Inflammation

Acute and chronic inflammation are distinct patterns with different dominant cells, timing, and outcomes.

Acute inflammation is rapid in onset and short in duration. It is characterized by exudation of fluid and plasma proteins, plus predominantly neutrophilic leukocyte accumulation. It is typically triggered by infections, tissue necrosis, foreign bodies, or immune reactions that act quickly.

Chronic inflammation is more prolonged. It features lymphocytes and macrophages, tissue destruction, and attempts at repair with angiogenesis and fibrosis. Chronic inflammation may follow acute inflammation or begin insidiously in settings like persistent infections, autoimmune disease, or prolonged toxin exposure.

Cardinal Signs and Systemic Effects

Clinically, acute inflammation is recognized by classic local signs: redness, heat, swelling, pain, and loss of function. Redness and heat result from increased blood flow, swelling from increased vascular permeability and fluid exudation, and pain from mediators that sensitize nerve endings and from the pressure of swelling. Loss of function reflects pain and structural disruption.

Systemic effects of inflammation are called acute phase reactions. Key components include fever, leukocytosis or leukopenia, increased acute phase proteins, and sometimes sepsis or shock in severe cases. Fever arises from pyrogens that increase prostaglandin E2 in the hypothalamus and reset the temperature set point. Leukocytosis involves an increased white blood cell count, often neutrophilia in bacterial infections, lymphocytosis in viral infections, and eosinophilia in parasitic or allergic conditions. Acute phase proteins are produced by the liver and include C reactive protein, fibrinogen, and serum amyloid A. These have effects on opsonization, erythrocyte sedimentation rate, and immune regulation.

Acute Inflammation: Vascular Changes

Vascular events are an essential early component of acute inflammation. The first response is transient vasoconstriction of arterioles, followed soon by sustained vasodilation. This increases blood flow to the area and causes redness and warmth.

With vasodilation, microvascular permeability increases. Plasma proteins escape from the intravascular space into the interstitium. This process transforms the microcirculation from a normal laminar flow to one with stasis, where blood flow slows and red blood cells become more concentrated.

Increased vascular permeability can arise from multiple mechanisms. The most important early mechanism is endothelial cell contraction, which widens intercellular gaps, particularly in postcapillary venules. This is often mediated by histamine, bradykinin, leukotrienes, and other mediators and is usually rapid and reversible. Direct endothelial injury can also increase permeability, especially in severe burns, infections, or irradiation, and may lead to necrosis and detachment of endothelial cells with a more sustained leak. Leukocyte mediated endothelial injury can occur during the later phases of inflammation when activated leukocytes adhere to endothelium and release toxic oxygen species and proteases.

The net result is movement of protein rich fluid, called exudate, into the tissue. This is distinct from transudate, which is low protein fluid that results from altered hydrostatic or oncotic pressures without increased vascular permeability, as in heart failure or hypoalbuminemia.

A clear distinction between exudate and transudate is important in clinical practice.

Acute Inflammation: Cellular Events

White blood cell recruitment to sites of inflammation follows a regulated sequence within the microvasculature.

The first step is margination and rolling. With stasis, leukocytes move from the central axial flow to the periphery near the endothelium. Weak, transient adhesions form and break between leukocytes and endothelial cells so the cells roll along the vessel wall. Rolling is mediated by selectins. Endothelial cells upregulate E selectin and P selectin in response to cytokines. Leukocytes express ligands for these selectins.

Firm adhesion follows rolling. Cytokines such as TNF and IL 1 induce expression of adhesion molecules in the immunoglobulin superfamily on endothelial cells. Leukocyte integrins, which have been activated to high affinity states by chemokines, bind firmly to these Ig type molecules. This arrests leukocytes on the endothelial surface.

After firm adhesion, leukocytes undergo transmigration, also called diapedesis, across the endothelium, primarily in postcapillary venules. They pass through interendothelial spaces with the help of molecules such as PECAM 1, and then penetrate the basement membrane by secreting collagenases. Once in the extravascular space, leukocytes move toward the site of injury along chemotactic gradients.

Chemotaxis is guided by both exogenous and endogenous substances. Bacterial products act as exogenous chemoattractants. Endogenous chemoattractants include components of complement, especially C5a, arachidonic acid metabolites such as leukotriene B4, and certain cytokines and chemokines.

The nature of the infiltrating leukocytes changes with time and type of stimulus. Neutrophils dominate in the first 6 to 24 hours in many acute responses because they are more numerous in blood, respond rapidly to chemokines, and adhere well to adhesion molecules. Later, monocytes and macrophages predominate, particularly after 24 to 48 hours. In viral infections, lymphocytes may be the first cells to arrive. In some hypersensitivity reactions, eosinophils are prominent.

Leukocyte Activation and Function

Once leukocytes arrive at the site of injury, they must be activated to perform their effector functions. Activation is triggered by recognition of microbes, necrotic cells, and mediators. It results in enhanced phagocytosis, production of reactive oxygen and nitrogen species, secretion of lysosomal enzymes, and release of various cytokines and mediators.

Phagocytosis is a central function of neutrophils and macrophages. It occurs in three steps. First, the target particle, such as a bacterium, is recognized and attached to the phagocyte surface. Opsonins, molecules that coat particles and enhance recognition, are critical in this step. Important opsonins are IgG, C3b, and certain lectins. Second, the particle is engulfed with extension of pseudopods that enclose it in a phagosome. Third, the phagosome fuses with lysosomes to form a phagolysosome, where killing and degradation occur.

Microbial killing in phagolysosomes relies on oxygen dependent and oxygen independent mechanisms. The most powerful system is the oxidative burst. Activation of NADPH oxidase converts oxygen to superoxide. This can be converted to hydrogen peroxide and, in the presence of myeloperoxidase and halide ions such as chloride, into hypochlorous radical, which is highly microbicidal. Oxygen independent mechanisms include lysosomal enzymes like defensins, lysozyme, and proteases.

If any of these phagocyte functions are defective, recurrent infections may occur. Specific genetic defects and their diseases are addressed elsewhere, but you should link patterns of infection to probable functional defects for USMLE questions.

Leukocytes also release mediators that amplify inflammation, recruit additional cells, and influence systemic responses. These mediators are tightly regulated, because excessive release of reactive oxygen species and proteases can damage host tissues and contribute to pathology such as abscess formation or chronic tissue injury.

Chemical Mediators of Inflammation

Chemical mediators orchestrate vascular and cellular events in inflammation. They can be derived from plasma or cells and are often produced or activated only in response to stimuli. Many mediators have short half lives and are rapidly inactivated or degraded.

Histamine is a key preformed mediator stored in mast cells and also in basophils and platelets. It is released in response to physical injury, immune reactions involving IgE, complement fragments such as C3a and C5a, and cytokines. Histamine causes arteriolar vasodilation and increased venular permeability through endothelial contraction.

Serotonin, stored in platelets and some neuroendocrine cells, has similar but less central roles in vascular effects.

Arachidonic acid metabolites, also known as eicosanoids, are generated from membrane phospholipids by phospholipase A2. Cyclooxygenase enzymes convert arachidonic acid to prostaglandins and thromboxanes. Lipoxygenases generate leukotrienes and lipoxins. Prostaglandins mediate vasodilation, pain, and fever. Thromboxane A2 causes vasoconstriction and promotes platelet aggregation. Leukotrienes increase vascular permeability and are potent chemoattractants. Lipoxins have anti inflammatory effects and help resolve inflammation.

Cytokines are proteins produced by many cells, especially activated macrophages and T lymphocytes, that modulate the inflammatory response. TNF and IL 1 are crucial for leukocyte recruitment and systemic acute phase reactions. They induce endothelial adhesion molecules, produce fever, stimulate hepatic acute phase protein production, and affect metabolism. Chemokines are a subset of cytokines that act as chemoattractants for specific leukocyte subsets.

Plasma protein derived mediators include complement, kinins, and products of the clotting system. Complement activation, via classical, alternative, or lectin pathways, generates C3a and C5a which increase vascular permeability and attract leukocytes, and C3b, which acts as an opsonin. C5b to C9 assemble into the membrane attack complex, which can lyse some microbes directly. The kinin system produces bradykinin, which increases vascular permeability, causes arteriolar dilation, and contributes to pain. Coagulation system activation generates fibrin and thrombin, and interactions with inflammation are bidirectional, as inflammatory mediators can promote clotting and clotting components can affect inflammation.

Nitric oxide, produced by endothelial cells and macrophages, causes vasodilation and has microbicidal as well as regulatory roles. Reactive oxygen species, generated by leukocytes, act as mediators that can increase vascular permeability and tissue damage when not adequately controlled.

Types and Patterns of Acute Inflammation

Acute inflammation can show different morphologic patterns depending on the tissue, the nature of the injury, and the severity.

Serous inflammation involves outpouring of a relatively cell poor fluid that may originate from plasma or from secretions of mesothelial cells. It is typically seen in effusions into body cavities such as the pleural, peritoneal, or pericardial spaces, and in skin blisters from burns or viral infections.

Fibrinous inflammation arises when vascular permeability is significant enough for large molecules like fibrinogen to escape the circulation. Fibrin is deposited in extracellular spaces, especially in linings like the pericardium or pleura. Fibrinous pericarditis, for example, can produce a roughened surface with a friction rub. If fibrin is not removed, it can lead to organization and fibrous scarring.

Suppurative or purulent inflammation is marked by the production of pus, a collection of neutrophils, necrotic cells, and edema fluid. Localized collections of pus in a tissue are called abscesses. Abscesses often arise from pyogenic bacteria and are characterized by central necrotic debris surrounded by a zone of preserved neutrophils and outer dilated vessels and fibroblasts.

Ulceration is a local defect in the surface of an organ or tissue that results from sloughing of inflamed necrotic tissue. It can occur in the mucosa of the mouth, stomach, intestines, or genitourinary tract, and on skin.

Recognition of these patterns in images or gross descriptions is important for exam questions that link morphology to pathophysiology.

Outcomes of Acute Inflammation

The outcome of acute inflammation depends on the nature and severity of the injury, the tissue involved, and the ability of the host to mount an appropriate response.

Complete resolution occurs when the injury is limited or short lived, there has been little tissue destruction, and the tissue can regenerate. In this case, exudate is removed and the structure and function of the tissue return to normal.

Healing by connective tissue replacement, or scarring, takes place when tissue destruction is substantial, parenchymal cells cannot regenerate, or there is abundant fibrin exudation that cannot be cleared. The inflammatory site is replaced by fibrous tissue.

Progression to chronic inflammation occurs when the acute response cannot eliminate the inciting agent or when there are repeated episodes of acute inflammation. Some stimuli start as low grade chronic inflammation without a clear acute phase.

Chronic Inflammation: Causes and Features

Chronic inflammation can follow unresolved acute inflammation or develop from the beginning in certain settings. Persistent infections, such as tuberculosis or some viral and fungal infections, can generate a chronic inflammatory response because the agent is difficult to eradicate and elicits a delayed type hypersensitivity reaction. Prolonged exposure to potentially toxic agents, such as silica or certain lipids, can also cause chronic inflammation. Autoimmune diseases are a key category where chronic inflammation arises from immune reactions against self antigens.

Histologically, chronic inflammation is characterized by infiltration with mononuclear cells, especially macrophages, lymphocytes, and plasma cells. There is tissue destruction driven by these inflammatory cells, and simultaneous attempts at healing, with proliferation of small blood vessels and fibroblasts that deposit collagen.

Macrophages are central in chronic inflammation. They derive from circulating monocytes and from embryonic tissue residents, and accumulate at sites of chronic injury. Activated macrophages release cytokines, growth factors, and enzymes that perpetuate inflammation and influence repair. Lymphocytes, especially T cells, interact with macrophages in a reciprocal way. T cells produce cytokines that activate macrophages, and macrophages present antigens and produce cytokines that activate T cells. This feedback loop can sustain chronic inflammation over long periods.

Granulomatous Inflammation

Granulomatous inflammation is a distinctive chronic inflammatory pattern. Its hallmark is the formation of granulomas, which are collections of activated macrophages, often with a modified epithelial like appearance, known as epithelioid cells, surrounded by a collar of lymphocytes. Sometimes multinucleated giant cells formed by the fusion of macrophages are present.

Granulomatous inflammation usually occurs when the immune system attempts to wall off substances that are difficult to eliminate, such as certain bacteria, fungi, parasites, foreign bodies, or persistent antigens. The formation of granulomas often involves T cell mediated immune responses.

Types of granulomas can be broadly distinguished by their cause and appearance. Immune granulomas arise in response to persistent T cell mediated immune activation, as seen in tuberculosis, sarcoidosis, some fungal infections, and reaction to certain self antigens. Foreign body granulomas form around inert exogenous material that is too large to be phagocytosed by a single macrophage, such as sutures or talc. Multinucleated giant cells in foreign body granulomas often have nuclei arranged haphazardly, while in some immune granulomas like those in tuberculosis, so called Langhans type giant cells may have peripheral nuclear arrangements. Caseating granulomas contain a central area of amorphous, granular, eosinophilic necrosis, typically seen in tuberculosis. Noncaseating granulomas lack this central necrosis and are characteristic of many other conditions such as sarcoidosis, although exceptions and overlaps exist.

On USMLE style questions, you will often be asked to identify a granulomatous pattern on histology or description, then link it to likely diagnoses, to specific infectious agents, or to a type of immune response.

Systemic Effects and Complications of Chronic Inflammation

Chronic inflammatory states can have prominent systemic effects. Persistent elevations of acute phase reactants, alterations in cytokine profiles, and ongoing immune activation can contribute to anemia of chronic disease, cachexia, and secondary amyloidosis. Anemia of chronic disease arises from cytokine mediated changes in iron metabolism and reduced erythropoietin response. Cachexia, especially in the setting of malignancy, is strongly associated with TNF and other cytokines that alter appetite and metabolism. Secondary amyloidosis involves deposition of amyloid derived from serum amyloid A, which is increased in chronic inflammatory states.

Locally, persistent inflammation with ongoing tissue destruction and repair can lead to architectural distortion, fibrosis, and loss of organ function. Chronic inflammation can contribute to the pathogenesis of many cancers by increasing cell turnover, generating reactive oxygen species that cause DNA damage, and providing a microenvironment conducive to tumor growth.

Resolution and Repair

Inflammation does not persist indefinitely in most situations. The process of resolution involves active termination of the inflammatory response and transition to repair. Mediators that promote inflammation are degraded or dissipate. Anti inflammatory cytokines and lipid mediators, including lipoxins and specialized pro resolving mediators, help shift the balance away from leukocyte recruitment and activation. Macrophages play a key role by phagocytosing apoptotic neutrophils, clearing debris, and producing mediators that promote tissue repair and fibrosis when necessary.

Repair processes involve cell proliferation, angiogenesis, and deposition of extracellular matrix, topics that are explored in detail in discussions of healing and repair in general pathology. In the context of inflammation, you should understand that the same mediators and cells that participate in inflammatory reactions often have dual roles in initiating or modulating repair.

Laboratory Markers of Inflammation

In clinical and exam scenarios, inflammation is frequently inferred from laboratory markers rather than directly observed. Elevated C reactive protein and erythrocyte sedimentation rate are nonspecific but sensitive indicators of systemic inflammation, reflecting increased acute phase proteins. Leukocyte counts are commonly altered, with patterns that suggest bacterial, viral, or allergic etiologies depending on which cell type is predominant. In chronic inflammatory or autoimmune conditions, specific autoantibodies or cytokine levels may be measured to assess disease activity.

Understanding these markers allows you to connect the described laboratory profile in a question stem to an underlying inflammatory process and to distinguish acute from chronic patterns where appropriate.

Summary

Inflammation is a coordinated vascular and cellular response to injury or infection that serves to eliminate the cause of damage, clear necrotic tissue, and initiate repair. Acute inflammation is rapid and neutrophil rich, with key features of vasodilation, increased vascular permeability, and exudate formation. Chronic inflammation is prolonged, with mononuclear cells, tissue destruction, and fibrosis. Chemical mediators, including histamine, eicosanoids, cytokines, complement, and kinins, regulate nearly every step. Distinct morphologic patterns such as serous, fibrinous, purulent, and granulomatous inflammation provide clues to underlying causes. For USMLE purposes, linking clinical, morphologic, and laboratory findings to specific inflammatory mechanisms is essential.