Acute inflammation mechanisms

Acute inflammation mechanisms

Acute inflammation mechanisms

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Acute Inflammation - The First Responders

Acute inflammation initiation and resolution

  • Triggers: Microbial Pathogen-Associated Molecular Patterns (PAMPs) or host-derived Damage-Associated Molecular Patterns (DAMPs).
  • Sensors: Sentinel cells like macrophages and mast cells use Pattern Recognition Receptors (e.g., Toll-like receptors - TLRs).
  • Amplification: The inflammasome, a cytosolic complex, activates caspase-1, leading to the maturation and release of IL-1β.

⭐ The inflammasome-driven production of IL-1β is a critical step, making IL-1 a key therapeutic target in several autoinflammatory syndromes.

Vascular Changes - The Floodgates Open

  • Vasodilation: Arterioles expand, causing ↑ blood flow.
    • Mediators: Histamine, bradykinin, Nitric Oxide.
    • Cardinal Signs: Rubor (redness) & Calor (heat).
  • ↑ Vascular Permeability: Endothelial cell contraction opens gaps, allowing protein-rich exudate to leak into tissues.
    • Result: Tumor (swelling/edema) & ↑ interstitial fluid.
    • Dolor (pain) is mediated by bradykinin and prostaglandins.

Fluid movement is governed by Starling forces: $P_{net} = [(P_c - P_i) - \sigma(\pi_c - \pi_i)]$

Exudate (inflammatory) is protein-rich (sp. gr. > 1.020) and cellular, unlike transudate (hydrostatic), which is a protein-poor ultrafiltrate.

Vascular changes in acute inflammation

Cellular Events - Leukocyte Invasion

Leukocytes journey from the vessel lumen to the interstitial tissue in a defined sequence to fight infection and clear debris. This multi-step process is mediated by specific adhesion molecules and chemokines.

Leukocyte Extravasation and Phagocytosis

  • Margination & Rolling: Slow blood flow pushes leukocytes to the periphery. Endothelial E/P-selectins loosely bind to leukocytes.
  • Adhesion: Integrins on leukocytes bind firmly to endothelial ICAM-1. 📌 Mnemonic: Selectins for Slowing/rolling; Integrins for Immobile adhesion.
  • Transmigration (Diapedesis): Leukocytes squeeze between endothelial cells, mediated by PECAM-1 (CD31).
  • Chemotaxis: Cells migrate toward inflammatory stimuli like $C5a$, $LTB_4$, and IL-8.

⭐ In Leukocyte Adhesion Deficiency (LAD), defective integrins lead to recurrent bacterial infections without pus formation and delayed separation of the umbilical cord.

Phagocytosis - The Clean-Up Crew

  • Opsonization: Pathogens are tagged for destruction by opsonins, primarily IgG and C3b.
  • Engulfment & Destruction: Phagocytes engulf tagged pathogens into a phagosome. This fuses with a lysosome to form a phagolysosome, where destruction occurs.
  • Killing Mechanisms:
    • Respiratory Burst: The primary method. Utilizes oxygen to create reactive oxygen species. $O_2 \xrightarrow{NADPH \text{ oxidase}} O_2^{\cdot-} \xrightarrow{SOD} H_2O_2 \xrightarrow{MPO} HOCl$ (bleach)
    • NETs: Neutrophil Extracellular Traps are webs of DNA and proteins released to trap and kill pathogens.

Chronic Granulomatous Disease (CGD) results from a defective NADPH oxidase, leading to recurrent infections by catalase-positive organisms.

Phagocytosis: Recognition, Engulfment, Digestion, Exocytosis

Chemical Mediators - The Conductors

Orchestrate inflammation from two sources:

  • Cell-Derived (Ready or made on demand):

    • Histamine: Vasodilation, ↑ vascular permeability.
    • Arachidonic Acid Metabolites:
      • Prostaglandins (via COX): Pain, fever.
      • Leukotrienes (via Lipoxygenase): Chemotaxis, bronchospasm.
    • Cytokines: TNF, IL-1, IL-6 drive systemic effects (fever).
  • Plasma-Derived (From liver):

    • Complement: C3a/C5a (inflammation), C3b (opsonization).
    • Kinin System: Bradykinin (pain, vasodilation).

Arachidonic Acid Pathway: COX, LOX, and CYP450

⭐ The cytokine triad of TNF, IL-1, and IL-6 drives the systemic acute-phase response, causing fever and hepatic synthesis of proteins like C-reactive protein (CRP).

High‑Yield Points - ⚡ Biggest Takeaways

  • Key vascular changes are vasodilation (redness, heat) and increased permeability (swelling), driven by histamine and bradykinin.
  • Neutrophil extravasation is a cascade: rolling (selectins), adhesion (integrins), and transmigration (PECAM-1).
  • Chemotaxis is crucial for recruitment, guided by potent chemoattractants like C5a, LTB4, and IL-8.
  • Pain is mediated by bradykinin and PGE₂; fever is driven by cytokines like TNF and IL-1.
  • Phagocytosis relies on opsonization by IgG and C3b to clear pathogens and debris effectively.
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Practice Questions: Acute inflammation mechanisms

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A 24-year-old woman comes to the physician because of progressively worsening episodes of severe, crampy abdominal pain and nonbloody diarrhea for the past 3 years. Examination of the abdomen shows mild distension and generalized tenderness. There is a fistula draining stool in the perianal region. Immunohistochemistry shows dysfunction of the nucleotide oligomerization binding domain 2 (NOD2) protein. This dysfunction most likely causes overactivity of which of the following immunological proteins in this patient?

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Flashcards: Acute inflammation mechanisms

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Leukocyte adhesion deficiency is characterized by _____ neutrophils at infection sites

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Leukocyte adhesion deficiency is characterized by _____ neutrophils at infection sites

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