Comprehensive Study Notes on Inflammation: Acute, Chronic, and Systemic Responses

Introduction to Inflammation

Definition: Inflammation is a response (reaction) of living vascularized tissues to endogenous and exogenous stimuli that cause cell injury.
Nature of the Response:
- It is fundamentally a protective response.
- It may be harmful in some situations (e.g., hypersensitivity).
- It is closely intertwined with the process of tissue repair and the healing process.
Purpose: It serves to rid the body of both the initial cause of cell injury (e.g., microbes, toxins) and the consequences of such injury (e.g., necrotic cells and tissues).
Consequences of Absence: Without inflammation, infections would go unchecked, wounds would never heal, and injured tissues might remain permanent.
Harms of Inflammation: The reaction can become the cause of disease when excessive or inappropriate:
- Autoimmune diseases.
- Hypersensitivity reactions.
- Systemic Inflammatory Response Syndrome (SIRS).
- Defective Inflammation: This is also responsible for serious illness.

Comparison of Acute and Chronic Inflammation

Classification Criteria: Inflammation is classified based on the duration of the lesion and histologic appearances.
Acute Inflammation:
- Duration: Relatively short (hours to days).
- Characteristics: Exudation of fluid and plasma proteins; predominantly neutrophilic (PMN) infiltration.
- Onset: Fast (minutes or hours).
- Tissue Injury: Usually mild and self-limited.
- Local/Systemic Signs: Prominent.
Chronic Inflammation:
- Duration: Longer (days to years).
- Characteristics: Infiltration with mononuclear cells (macrophages, lymphocytes, plasma cells), vascular proliferation (angiogenesis), and scarring (fibrosis).
- Onset: Slow (days).
- Tissue Injury: May be severe and progressive.
- Local/Systemic Signs: Less prominent.

Sequential Steps of the Inflammatory Reaction

Recognition: Identifying the noxious agent or initiating stimulus.
Recruitment: Bringing leukocytes and plasma proteins from the blood into the tissues.
Removal: Eliminating the stimulus for inflammation.
Regulation: Controlling the response to prevent excessive damage.
Repair: A series of events that heal the damaged tissue.

Recognition of Microbes and Damaged Cells

Cellular Receptors for Microbes:
- Toll-like Receptors (TLRs): Located in plasma membranes and endosomes. They recognize Pathogen-Associated Molecular Patterns (PAMPs).
- Function: Stimulates the production and expression of secreted and membrane proteins.
Sensors of Cell Damage:
- Cytosolic receptors recognize Damage-Associated Molecular Patterns (DAMPs).
- DAMP Examples: Uric acid, ATP, reduced intracellular $K^+$ concentrations, and extranuclear DNA.
- Inflammasome: A multiprotein cytosolic complex activated by receptors. It induces the production of the cytokine Interleukin-1 (IL-1).
- Autoinflammatory Syndromes: Caused by gain-of-function mutations in these cytosolic receptors.
Circulating Proteins:
- Mannose-binding Lectin: Recognizes microbial sugars; promotes ingestion (opsonization) and activates the complement system.
- Collectins: Bind to microbes and promote phagocytosis.

Acute Inflammation: Causes and Major Components

Causes:
- Infections (bacterial, viral, fungal, parasitic).
- Trauma (physical and chemical agents).
- Tissue necrosis (e.g., from ischemia).
- Foreign bodies (e.g., splinters, sutures).
- Immune reactions (hypersensitivity).
Major Components:
1. Vasodilation.
2. Increased vascular permeability.
3. Emigration of leukocytes from microcirculation and their activation.

Terminology and Fluid Dynamics

Edema: Excess fluid in interstitial or serous cavities; can be exudate or transudate.
Exudation: The escape of fluid, proteins, and blood cells from the vascular system into interstitial tissue.
Exudate:
- Inflammatory extravascular fluid.
- High protein concentration and cellular debris.
- Specific gravity above $1.020$ .
- Implies an alteration in normal permeability of small blood vessels.
Pus (Purulent Exudate): An inflammatory exudate rich in leukocytes (mostly neutrophils), debris, and microbes.
Transudate:
- Low protein content and few cells.
- Specific gravity less than $1.020$ .
- Results from osmotic or hydrostatic imbalance (e.g., heart failure, liver disease) without increased vascular permeability.

Vascular Response and Leakiness Mechanisms

Vascular Events:
1. Brief arteriolar vasoconstriction.
2. Vasodilation (induced by histamine, nitric oxide, bradykinin): Accounts for warmth (calor) and redness (rubor).
3. Stasis: Slower blood flow as dilated vessels become packed with red cells.
Mechanisms of Vascular Permeability:
1. Endothelial Cell Contraction: Most common. Widens intercellular gaps in venules. Induced by histamine, bradykinins, leukotrienes (Immediate transient response: $15 \text{--} 30\,\text{min}$ ). Cytokines (TNF, IL-1) cause junction retraction ( $4 \text{--} 6\,\text{hrs}$ post-injury, lasting over $24\,\text{hrs}$ ).
2. Direct Endothelial Injury: Caused by severe injuries (burns, toxins).Leads to necrosis and detachment. Immediate sustained response (affects all microcirculation levels) or delayed prolonged leakage ( $2 \text{--} 12\,\text{hrs}$ later).
3. Leukocyte-Mediated Injury: Adherent leukocytes release toxic oxygen radicals and proteolytic enzymes, damaging endothelium.
4. Increased Transcytosis: Mediated by VEGF via intracellular vesicles.
5. Angiogenesis: Leakage from new, immature blood vessels.

Cellular Response: Leukocyte Extravasation

Extravasation Sequence:
1. Margination: Accumulation of neutrophils along endothelial surfaces (peripheral zone) due to hemodynamic changes.
2. Rolling: Leukocytes tumble and adhere transiently. Mediated by the Selectin family:
  - L-selectin: On most leukocytes.
  - E-selectin: On endothelial cells.
  - P-selectin: On endothelium and platelets.
  - Selectins bind sialylated oligosaccharides (e.g., Sialyl-Lewis X).
3. Adhesion: Firm binding to endothelium. Regulated by:
  - Integrins: On leukocytes (VLA-4, LFA-1).
  - Immunoglobulin Superfamily: On endothelial cells (ICAM-1, VCAM-1).
4. Transmigration (Diapedesis): Squeezing between endothelial cells at junctions. Mediated by PECAM-1 (CD31). Leukocytes use collagenases to cross the basement membrane.
5. Chemotaxis: Unidirectional migration toward a chemical gradient. Important factors:
  - Exogenous: Bacterial products.
  - Endogenous: Complement $C5a$ , Leukotriene $B_4$ ( $LTB_4$ ), and Cytokines (e.g., $IL\text{-}8$ ).

Phagocytosis and Killing

Leukocyte Activation: Triggered by microbes and necrotic products; results in increased cytosolic $Ca^{2+}$ and activation of enzymes (PKC, Phospholipase $A_2$ ).
Phagocytosis Steps:
1. Recognition and Attachment: Facilitated by Opsonins (IgG Fc portion, $C3b$ , collectins). Receptors include mannose and scavenger receptors.
2. Engulfment: Pseudopodia enclose the particle into a phagosome, which fuses with a lysosome to form a phagolysosome.
3. Killing and Degradation:
  - Reactive Oxygen Species (ROS): NADPH oxidase converts oxygen to superoxide ( $O_2^{\cdot -}$ ), then to hydrogen peroxide ( $H_2O_2$ ).
  - Myeloperoxidase (MPO): Converts $H_2O_2$ and $Cl^-$ to HOCl (hypochlorous radical), a potent antimicrobial.
  - Reactive Nitrogen Species: Derived from Nitric Oxide (NO).
Granule Constituents:
- Major Basic Protein: Toxic to parasites (eosinophils).
- Defensins: Punch holes in membranes.
- Lysozyme: Degrades bacterial coat sugars.

Defects in Leukocyte Function

Leukocyte Adhesion Deficiency (LAD):
- LAD-1: Lack of integrins; leads to impaired adhesion and phagocytosis.
- LAD-2: Absence of Sialyl-Lewis X.
Chronic Granulomatous Disease (CGD): Inherited defects in NADPH oxidase; leads to recurrent bacterial infections (lungs, nares, gingivitis).
Chediak-Higashi Syndrome: Autosomal recessive; disordered intracellular trafficking. Characterized by neutropenia, delayed killing, giant granules, and recurrent infections.
Acquired Defects: Can occur in diabetes mellitus, thermal injuries, sepsis, malnutrition, and due to certain drugs.

Chemical Mediators of Inflammation

General Characteristics:
- Produced in response to stimuli (microbes, necrotic cells).
- Most bind to specific receptors; some have direct enzymatic activity.
- Short-lived: decay quickly, inactivated enzymatically, or scavenged.
Vasoactive Amines:
- Histamine: Main source is mast cells. Stimulated by trauma, IgE, anaphylatoxins ( $C3a$ , $C5a$ ), neuropeptides ( $Substance\,P$ ). Causes arteriolar dilation and increased venular permeability via $H_1$ receptors.
- Serotonin (5-HT): Found in platelets and enterochromaffin cells. Acts as a neurotransmitter and vasoconstrictor.
Arachidonic Acid (AA) Metabolites (Eicosanoids):
- Release: AA is released from membrane phospholipids by Phospholipase $A_2$ .
- Cyclooxygenase (COX) Pathway: Produces Prostaglandins.
  - COX-1: Constitutively expressed; homeostatic (GIT, kidneys).
  - COX-2: Induced by inflammation.
  - $PGE_2$ : Fever, pain, vasodilation.
  - $PGI_2$ (Prostacyclin): Vasodilation, inhibits platelet aggregation.
  - $TXA_2$ (Thromboxane): Vasoconstriction, promotes platelet aggregation.
  - $PGD_2$ : Vasodilation, edema, neutrophil chemoattractant.
- Lipoxygenase Pathway: Produces Leukotrienes and Lipoxins.
  - $LTB_4$ : Chemotactic agent, neutrophil activator.
  - $LTC_4$ , $LTD_4$ , $LTE_4$ : Vasoconstriction, bronchospasm, increased permeability.
  - Lipoxins ( $LXA_4$ , $LXB_4$ ): Suppress inflammation by inhibiting leukocyte recruitment.

Cytokines and Chemokines

TNF and IL-1: "Master Cytokines" produced by macrophages and dendritic cells.
- Local Effects: Endothelial activation (selectin expression), procoagulant activity.
- Systemic Effects: Fever, metabolic abnormalities, hypotension (shock).
- TNF specific: Promotes lipid/protein catabolism; contributes to cachexia.
IL-6: Systemic acute-phase response.
Chemokines: Small proteins classified by cysteine arrangement:
- C-X-C (e.g., IL-8/CXCL8): Acts mainly on neutrophils.
- C-C (e.g., MCP-1, Eotaxin): Recruits monocytes, eosinophils, lymphocytes.
- C: Specific for lymphocytes.
- CX3C (Fractalkine): Strong adhesion for monocytes/T cells.
- Function: Inflammatory (recruitment) and Homeostatic (tissue architecture).

Plasma-Derived Mediators

Complement System: Defensive enzymatic cascade. Critical step is $C3$ cleavage.
- Classical Pathway: Triggered by antigen-antibody complex (IgM or IgG).
- Alternative Pathway: Triggered by microbial surfaces (LPS, polysaccharides).
- Lectin Pathway: Mannose-binding lectin binds microbial sugars.
- Products: $C3a$ , $C5a$ (Inflammation, anaphylatoxins); $C3b$ (Opsonization); $C5b\text{--}9$ (Membrane Attack Complex/MAC for cell lysis).
- Regulators: $C1\,inhibitor$ (deficiency causes hereditary angioedema), $DAF$ and $CD59$ (deficiency in $PNH$ causes red cell lysis), $Factor\,H$ .
Kinins: Derived from High Molecular Weight Kininogen (HMWK). Bradykinin causes permeability, smooth muscle contraction, vasodilation, and pain.
Clotting System: $Factor\,Xa$ (permeability), Thrombin (adhesion), Fibrinopeptides (chemotaxis).

Morphologic Patterns of Acute Inflammation

Serous Inflammation: Exudation of cell-poor, watery fluid. Accumulation in cavities is an effusion (e.g., skin blister, viral infection).
Fibrinous Inflammation: Occurs with large vascular leaks or procoagulant stimuli (cancer). Fibrinogen passes out and forms fibrin. Characteristic of body cavity linings (pericardium). Can lead to organization and scarring.
Suppurative (Purulent) Inflammation: Production of pus (neutrophils, debris, edema). Caused by pyogenic bacteria (e.g., staphylococci).
- Abscess: Circumscribed accumulation of pus within a tissue, often with a central necrotic region and a pyogenic membrane.
Ulcers: Local defect/excavation of a surface produced by sloughing of inflamed necrotic tissue. Common in GIT mucosa and skin of lower extremities.
Catarrhal: Mild inflammation of mucous membranes (e.g., Rhinitis).
Pseudomembranous: Extensive confluent necrosis and fibrinopurulent layer over mucosa (e.g., Diphtheria, C. difficile colitis).

Outcomes of Acute Inflammation

Complete Resolution: Normal structure restored. Happens when injury is short-lived with little destruction.
Healing by Connective Tissue Replacement (Scarring/Fibrosis): Occurs after substantial destruction, in non-regenerative tissues, or with abundant fibrin (Organization).
Progression to Chronic Inflammation: Occurs when the response cannot be resolved due to persistent injury or healing interference.

Chronic Inflammation

Definition: Prolonged process (weeks/months) where inflammation, destruction, and repair proceed simultaneously.
Causes: Persistent infections (TB, leprosy), prolonged exposure to toxins (silica, asbestos, lipids in atherosclerosis), and autoimmunity.
Cells of Chronic Inflammation:
- Macrophages: Dominant cells. Classically activated (M1) via IFN- $\gamma$ /microbes (microbicidal, pro-inflammatory) or Alternatively activated (M2) via IL-4/IL-13 (repair, anti-inflammatory).
- Lymphocytes: $Th1$ (IFN- $\gamma$ for M1), $Th2$ (IL-4/5/13 for eosinophils/M2), $Th17$ (IL-17 for PMNs).
- Eosinophils: IgE-mediated reactions and parasites. Contain major basic protein.
- Mast Cells: Release cytokines for both acute and chronic phases.

Granulomatous Inflammation

Definition: A form of chronic inflammation characterized by collections of activated macrophages (epithelioid cells) and T lymphocytes.
Types:
- Foreign Body Granuloma: Incited by inert materials (sutures, talc).
- Immune Granuloma: Caused by persistent microbes (TB) inducing a T-cell response.
Morphology:
- Epithelioid cells: Macrophages with abundant cytoplasm.
- Langhans Giant Cells: Fused macrophages with peripheral nuclei.
- Caseating Granuloma: Central cheese-like necrosis due to hypoxia/free radicals (e.g., Tuberculosis).
- Non-caseating: No central necrosis (e.g., Sarcoidosis, Crohn disease).

Systemic Effects of Inflammation (Acute Phase Response)

Mediators: TNF, IL-1, IL-6.
Fever: Cytokines stimuate PGE2 production in the hypothalamus to reset the temperature set-point.
Leukocytosis: Cytokines stimulate bone marrow production. Levels may reach $40,000 \text{--} 100,000\,\text{cells/}\mu\text{l}$ (Leukemoid reaction). "Shift to the left" refers to immature neutrophils.
Acute Phase Proteins (Liver-derived):
- C-Reactive Protein (CRP): Opsonin, fixes complement. High levels associated with coronary artery disease.
- Serum Amyloid A (SAA): Can cause secondary amyloidosis in chronic inflammation.
- Fibrinogen: Causes Red Blood Cells to stack (rouleaux), increasing the Erythrocyte Sedimentation Rate (ESR).
- Hepcidin: Iron-regulating peptide; chronically high levels cause the anemia of chronic inflammation.
Clinical Signs: Increased HR/BP, rigors, chills, anorexia, somnolence, and malaise.
Septic Shock: Fall in BP, DIC, and metabolic abnormalities induced by high TNF levels in severe infections.