Chronic Inflammation

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Introduction to Chronic Inflammation

Defining Chronic Inflammation

Chronic inflammation is defined as a physiological response of prolonged duration—lasting for weeks or months (and sometimes even years). Unlike acute inflammation, which is a rapid, short-lived response characterized by fluid leakage and neutrophil infiltration, chronic inflammation is entirely different in its nature.

The defining characteristic of chronic inflammation is that three distinct processes occur simultaneously at the site of the lesion:

1. Active Inflammation: The immune system continues to fight.
2. Tissue Injury: The ongoing battle causes collateral damage to the host's own tissues.
3. Attempts at Repair: The body simultaneously tries to heal the damage, usually through scarring (fibrosis) and the creation of new blood vessels.

Onset and Progression

Chronic inflammation does not always start the same way. It can initiate in two main ways:

• Following Acute Inflammation: If an initial acute inflammatory response fails to clear the offending agent (like a persistent bacterial infection), the body transitions the response into a chronic state.
• Insidious (Sneaky) Onset: It may begin as a low-grade, smoldering response from the very beginning. In these cases, there are absolutely no manifestations of a preceding acute reaction. The patient might not even realize it is happening until significant tissue damage has already occurred (e.g., in rheumatoid arthritis or atherosclerosis).

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Causes of Chronic Inflammation

The immune system generally wants to clear a threat quickly and return to normal. Chronic inflammation only occurs when an offending agent is highly stubborn or when the immune system becomes misdirected.

A. Persistent Infections

Certain microorganisms are extremely difficult for the immune system to eradicate. They possess unique defense mechanisms that allow them to survive inside the body for long periods. Examples include:

• Mycobacteria: The bacteria responsible for Tuberculosis and Leprosy. They have waxy cell walls that resist being digested by immune cells.
• Treponema pallidum: The spiral-shaped bacterium that causes Syphilis.
• Certain Viruses, Fungi, and Parasites: These pathogens can hide within host cells or evade immune detection, leading to a constant, unresolved immune battle.

B. Immune-Mediated Inflammatory Diseases (Hypersensitivity)

Sometimes, the immune system inappropriately attacks the body's own tissues or harmless environmental substances. Because the eliciting antigens (the targets of the attack) cannot be eliminated—since they are part of the body itself or constantly present in the environment—these disorders tend to be chronic and highly intractable (hard to treat).

• Autoimmune diseases: Conditions like Rheumatoid Arthritis or Systemic Lupus Erythematosus.
• Allergic diseases: Chronic asthma is a prime example of continuous inflammation driven by environmental antigens.

C. Prolonged Exposure to Potentially Toxic Agents

Continuous exposure to toxic substances over a long period forces the body into a state of chronic inflammation. These agents can be from outside the body (exogenous) or inside the body (endogenous).

• Exogenous Materials (Outside the body): An example is inhaled particulate silica. When inhaled, silica dust cannot be degraded by the lungs' immune cells. The constant presence of these sharp, indestructible particles induces a chronic inflammatory response known as silicosis.
• Endogenous Agents (Inside the body): An example is chronically elevated plasma lipid components (high blood cholesterol). The persistent presence of toxic oxidized lipids in the blood vessel walls triggers a relentless inflammatory response that contributes heavily to the development of atherosclerosis (hardening of the arteries).

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Morphologic Features (What it looks like under a microscope)

If a pathologist examines a tissue sample experiencing chronic inflammation under a microscope, they will observe three hallmark features that clearly distinguish it from acute inflammation.

1. Infiltration with Mononuclear Cells: Acute inflammation is dominated by polymorphonuclear cells (neutrophils). In contrast, chronic inflammation is dominated by mononuclear cells—cells with a single, round, or un-lobed nucleus. These specifically include macrophages, lymphocytes, and plasma cells.
2. Tissue Destruction: There is visible damage and destruction of the normal tissue architecture. This destruction is induced directly by the persistent offending agent (like a toxin or virus) or indirectly as collateral damage caused by the highly potent chemicals released by the accumulating inflammatory cells.
3. Attempts at Healing: Because the tissue is being destroyed, the body desperately tries to patch the holes. This connective tissue replacement of damaged tissue is accomplished by two simultaneous processes:
   • Angiogenesis: The sprouting and creation of new, fragile blood vessels to bring nutrients to the healing area.
   • Fibrosis: The massive deposition of fibrous connective tissue (collagen) by fibroblasts, resulting in heavy scarring.

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Cells and Mediators of Chronic Inflammation

The chronic inflammatory response is orchestrated by a specific cast of cellular characters. The major players include Macrophages, Lymphocytes, Plasma cells, Eosinophils, Neutrophils (in specific ongoing scenarios), and Mast cells.

A. Macrophages: The Dominant Cells

Macrophages are the undisputed heavyweights and dominant cells of chronic inflammation. They are large, highly capable tissue cells derived from circulating blood white blood cells called monocytes. After monocytes emigrate from the bloodstream into the tissues, they transform into macrophages.

Lifecycle and Activation of Macrophages

• Migration: The half-life of circulating blood monocytes is very short, about 1 day. Under the influence of chemical signals (adhesion molecules and chemotactic factors), they begin to migrate to a site of injury within 24 to 48 hours after the onset of acute inflammation.
• Transformation: When monocytes reach the extravascular tissue, they undergo a transformation into larger macrophages. These new tissue macrophages have much longer half-lives and a significantly greater capacity for phagocytosis (eating debris) than their blood monocyte precursors.
• Activation: To fight tough infections, macrophages must become "activated." This results in an increased cell size, an increased content of deadly lysosomal enzymes, a more active metabolism, and a much greater ability to kill ingested organisms.
• Epithelioid Cells: By light microscopy, these activated macrophages appear large, flat, and pink (when stained with H&E). Because this appearance makes them look very similar to squamous epithelial cells (skin-like cells), these highly activated macrophages are sometimes called epithelioid cells.

Macrophage Activation Signals and Secreted Products

Macrophages do not activate themselves; they require specific activation signals, which include:

• Bacterial endotoxins and other microbial products.
• Cytokines secreted by sensitized T lymphocytes, in particular the highly potent cytokine IFN-γ (Interferon-gamma).
• Various mediators produced during the acute inflammation phase.
• Extracellular Matrix (ECM) proteins such as fibronectin.

Once activated, macrophages become secretory factories. They secrete a wide variety of biologically active products. If left unchecked, these very products cause the severe tissue injury and fibrosis characteristic of chronic inflammation. These products include:

• Acid and neutral proteases: Enzymes that literally digest and break down proteins and tissue matrix.
• Plasminogen activator and other enzymes: These greatly amplify the generation of further proinflammatory substances.
• ROS (Reactive Oxygen Species) and NO (Nitric Oxide): Highly toxic free radicals meant to destroy bacteria, but which also heavily damage host cells.
• Arachidonic Acid (AA) metabolites: Known as eicosanoids (prostaglandins and leukotrienes) that sustain inflammation.
• Cytokines: Specifically IL-1 (Interleukin-1) and TNF (Tumor Necrosis Factor), which recruit even more immune cells.
• Growth factors: These chemicals influence the proliferation of smooth muscle cells and fibroblasts, directly driving the overproduction of ECM (scar tissue).

The Fate of Macrophages

After the initiating stimulus is finally eliminated and the inflammatory reaction abates, macrophages eventually die off or wander away into the lymphatic vessels to be cleared. However, in chronic inflammatory sites, macrophage accumulation persists, and the macrophages can even proliferate directly at the site. This steady accumulation is maintained by a continuous release of lymphocyte-derived chemokines that recruit and immobilize them.

If the battle is incredibly difficult, IFN-γ can induce several individual macrophages to fuse together into massive, multi-nucleated super-cells called Giant Cells.

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Granulomatous Inflammation

Granulomatous inflammation is a very specific, unique morphological pattern of chronic inflammation. It is a protective response essentially acting as a cellular quarantine.

Types of Granulomas

Granulomas are broadly divided into two main categories based on what incited their creation:

• Foreign Body Granulomas:
  • These are incited by completely inert (non-living, non-reactive) foreign bodies.
  • They induce inflammation in the absence of a T-cell mediated immune response (because the material does not present proteins for the T-cells to react to).
  • Causes include splinters, talc powder, large surgical sutures, or other indigestible fibers that are simply too large for a single macrophage to engulf via phagocytosis. The body walls them off instead.
• Immune Granulomas:
  • These are caused by a variety of biological agents (like specific bacteria or fungi).
  • These agents are fully capable of inducing a persistent, ongoing T-cell mediated immune response. The T-cells continuously release cytokines (like IFN-γ) that keep the macrophages gathered and highly activated.

Morphology: Components of a Granuloma

A fully formed granuloma is a microscopic structure consisting of several distinct layers and components:

• Epithelioid cells: The core is packed with activated macrophages that have changed shape to look like epithelial cells.
• Multinucleated giant cells: Frequently, epithelioid cells fuse to form massive giant cells (such as Langhans’ giant cells).
• Lymphocytes: A surrounding ring or collar of T-cells that constantly secrete cytokines to maintain the structure.
• Fibroblasts: Cells on the outermost rim laying down collagen to physically wall off the structure.
• Caseous necrosis: In certain diseases (like Tuberculosis), the very center of the granuloma dies and turns into a cheese-like, amorphous mass known as caseous necrosis.

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Causes and Examples of Granulomatous Inflammation

Many distinct agents can trigger this intense form of inflammation.

General Causes

• Bacterial: Tuberculosis (TB), Leprosy, Syphilis, Cat-scratch disease.
• Parasitic: Schistosomiasis, Leishmaniasis.
• Fungi: Histoplasmosis, Cryptococcosis, Coccidioides immitis.
• Inorganic Metals or Dusts: Silicosis (silica dust), Berylliosis (beryllium exposure).
• Foreign Body:
  • Endogenous (From inside): Keratin, Uric acid crystals (causing Gout), necrotic bone fragments.
  • Exogenous (From outside): Surgical sutures, splinters of wood.
• Drugs: Allopurinol, Sulphonamides.
• Unknown Etiology: Diseases where the body forms granulomas, but the exact trigger remains a medical mystery, such as Sarcoidosis and Crohn's disease.

Specific Disease Reactions (Detailed Breakdown)

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Types of Giant Cells

Giant cells are massive, multi-nucleated cells formed by the fusion of many individual cells. They are categorized based on whether they are part of normal, healthy bodily function (physiological) or the result of a disease process (pathological).