Assignment I - Complete Answers

Q1. Define Inflammation and Discuss in Detail the Cellular Events in Acute Inflammation

Definition

Cellular Events in Acute Inflammation

1. Dilation of small vessels leading to increased blood flow
2. Increased permeability of the microvasculature
3. Emigration of leukocytes from the microcirculation, accumulation at the site of injury, and activation

A. Vascular Changes

• Vasodilation - induced by histamine and other mediators acting on vascular smooth muscle. This is one of the earliest manifestations. It involves mainly postcapillary venules and results in increased blood flow causing heat and redness (erythema).
• Vasodilation is quickly followed by increased vascular permeability, with outpouring of protein-rich fluid into extravascular tissues.
• Loss of fluid and increased vessel diameter lead to stasis - slower blood flow and concentration of red cells in small vessels, causing vascular congestion.
• As stasis develops, blood leukocytes (mainly neutrophils) accumulate along the endothelium.

• Endothelial cell contraction forming intercellular gaps in postcapillary venules - the most common mechanism; triggered by histamine, bradykinin, leukotrienes, and substance P; occurs rapidly (within minutes) and is short-lived (reversible)
• Endothelial injury - direct damage causing endothelial necrosis and detachment (e.g., severe burns, bacterial toxins)
• Leukocyte-dependent endothelial injury - activated leukocytes release toxic oxygen species and enzymes that damage endothelial cells
• Increased transcytosis - transport of fluid via vesicular channels

B. Cellular Events - Leukocyte Recruitment

• P-selectin - on platelets and endothelium (released rapidly from Weibel-Palade bodies by histamine and thrombin)
• E-selectin - on endothelium (induced by TNF and IL-1)
• L-selectin - on leukocytes
• Selectins bind sialylated oligosaccharides (e.g., sialyl-Lewis X) on mucin-like glycoproteins

• Chemokines produced at the inflammatory site bind to receptors on rolling leukocytes, activating leukocyte integrins (e.g., LFA-1/CD11a-CD18, MAC-1/CD11b-CD18)
• These integrins bind to their endothelial ligands: ICAM-1 (Intercellular Adhesion Molecule-1) and VCAM-1 (Vascular Cell Adhesion Molecule-1)
• TNF and IL-1 upregulate ICAM-1 and VCAM-1 expression on endothelium

• Leukocytes migrate through the endothelium, primarily by paracellular route (between adjacent endothelial cells), driven by chemotactic gradients
• PECAM-1 (CD31), expressed on both leukocytes and endothelial cells, mediates the actual crossing step
• Leukocytes then traverse the basement membrane by secreting collagenases

• Bacterial products (e.g., formyl-methionyl-leucyl-phenylalanine, or f-Met-Leu-Phe)
• Complement component C5a
• Leukotriene B4 (LTB4)
• IL-8 (CXCL8) - a chemokine
• All bind to G-protein coupled receptors on leukocytes, trigger cytoskeletal assembly (actin polymerization), and direct movement

• Within the first 6-24 hours: neutrophils predominate (short-lived, respond rapidly to chemokines and bacterial products)
• After 24-48 hours: monocytes/macrophages predominate (longer-lived, more powerful phagocytes and cytokine producers)

C. Phagocytosis and Destruction of Injurious Agents

1. Recognition and attachment - via pattern recognition receptors (mannose receptor, scavenger receptors) and opsonin receptors. Opsonins (IgG, C3b, mannose-binding lectin) coat microbes and enhance phagocytosis.
2. Engulfment - cytoplasmic extensions flow around the particle, the plasma membrane pinches off to form a phagosome, which fuses with a lysosome to form a phagolysosome
3. Intracellular killing and degradation:
   • Reactive Oxygen Species (ROS) - produced by NADPH oxidase ("respiratory burst"); superoxide (O2-) is converted to hydrogen peroxide (H2O2) and then with myeloperoxidase + Cl- to hypochlorite (HOCl) - a potent microbicidal agent
   • Reactive Nitrogen Species - nitric oxide (NO) reacts with ROS to produce peroxynitrite (ONOO-), which kills microbes
   • Lysosomal enzymes - elastase, cathepsins, defensins, lysozyme, and bactericidal/permeability-increasing protein (BPI)

• Neutrophils also release extracellular fibrillar networks composed of nuclear chromatin decorated with granule proteins (histones, elastase, myeloperoxidase)
• NETs trap and kill extracellular bacteria and fungi

D. Outcomes of Acute Inflammation

1. Resolution - complete restoration of normal tissue if injury is limited and tissue has regenerative capacity
2. Abscess formation - if pus-forming (pyogenic) organisms are involved
3. Progression to chronic inflammation - if the agent persists or healing is defective
4. Scarring (fibrosis) - after substantial tissue destruction or fibrinous exudate

Q2. Short Note: Pathogenesis of Sickle Cell Anemia

Definition

Genetic Basis

• A single nucleotide substitution (GAG → GTG) in codon 6 of the beta-globin gene
• This replaces glutamic acid with valine at position 6 of the beta-globin chain
• Homozygotes (HbSS) have sickle cell disease; heterozygotes (HbAS) have sickle cell trait

Molecular Pathogenesis

• In the deoxygenated state, HbS molecules polymerize due to hydrophobic interactions (valine creates a hydrophobic "sticky patch")
• These HbS polymers form long, stiff rods (tactoids) that distort the RBC into a rigid sickle shape
• Initial sickling is reversible upon reoxygenation; however, repeated cycles of sickling cause membrane damage and result in irreversibly sickled cells (ISC)

• Decreased oxygen tension (hypoxia)
• Increased HbS concentration (dehydration)
• Decreased pH (acidosis)
• Increased 2,3-DPG
• Cold temperature
• Presence of HbF (fetal hemoglobin) is protective - it inhibits HbS polymerization

Consequences of Sickling

• Irreversibly sickled cells are rigid and fragile, with a shortened lifespan of 10-20 days (normal: 120 days)
• Extravascular hemolysis in spleen and intravascular hemolysis both occur
• Results in chronic hemolytic anemia (Hb typically 6-9 g/dL), jaundice, gallstones (pigment), splenomegaly (early), and reticulocytosis

• Sickled cells adhere abnormally to activated endothelium (mediated by adhesion molecules - VCAM-1, P-selectin, thrombospondin)
• Combined with activated neutrophils and platelets, this causes vascular occlusion
• Results in ischemia and infarction of multiple organs

• Acute episodes of ischemic pain in bones, joints, abdomen
• Dactylitis (hand-foot syndrome) in infants - infarction of metacarpal/metatarsal bones

• Spleen: Progressive splenic infarction → autosplenectomy → increased susceptibility to encapsulated organisms (Pneumococcus, Haemophilus)
• Bones: Avascular necrosis (femoral head, vertebrae)
• Kidneys: Renal papillary necrosis, sickle cell nephropathy
• Lungs: Acute chest syndrome (vaso-occlusion + infection/fat embolism); pulmonary hypertension
• Brain: Stroke (especially in children)
• Eyes: Proliferative retinopathy
• Liver: Hepatic sequestration crisis

• Transient arrest of erythropoiesis, most commonly due to Parvovirus B19 infection (infects erythroid progenitors)
• Sudden severe anemia

• Acute pooling of blood in spleen (or liver in adults) → rapid splenomegaly, hypovolemia
• Life-threatening

Laboratory Findings

• Peripheral smear: sickle-shaped cells, target cells, nucleated RBCs, reticulocytosis
• Hemoglobin electrophoresis: confirms HbSS (HbS ~90%, no HbA, increased HbF and HbA2)
• Sickling tests (sodium metabisulfite test, solubility test)

Q3. Short Note: Chemical Carcinogenesis

Definition

Historical Background

• First described by Percivall Pott (1775) who noted scrotal cancer in chimney sweeps - due to coal tar/soot
• Later, coal tar applied to rabbit ears experimentally produced skin cancer (Yamagiwa and Ichikawa, 1915)

Mechanism: Initiation and Promotion

• Results from exposure of cells to a sufficient dose of a carcinogenic agent
• Causes permanent, irreversible DNA damage (mutations)
• The cell is "initiated" but not yet cancerous
• Initiated cells have a selective growth advantage but are morphologically normal
• Initiators are highly reactive electrophiles (electron-deficient atoms) that react with nucleophilic (electron-rich) atoms in DNA - particularly guanine residues

• Promoters stimulate proliferation and clonal expansion of initiated cells
• Promoters are NOT carcinogenic by themselves; they are reversible (promotion is reversible if the promoter is withdrawn before sufficient mutations accumulate)
• Driven to proliferate, subclones of initiated cells sustain additional mutations and eventually a malignant clone emerges
• Examples of promoters: phorbol esters (TPA), hormones (estrogens), phenobarbital

Classification of Chemical Carcinogens

• Do NOT require metabolic conversion to become carcinogenic
• Most are weak carcinogens
• Examples: alkylating agents (cyclophosphamide, chlorambucil, nitrosoureas), acylating agents (dimethylcarbamoyl chloride)
• Important because some are cancer chemotherapy drugs - they can induce secondary malignancies (especially acute myeloid leukemia)

• Require metabolic activation to become carcinogenic; the active products are called ultimate carcinogens
• Most are metabolized by cytochrome P-450-dependent monooxygenases (polymorphic genes - susceptibility varies between individuals)
• Examples and their associated cancers:

Molecular Targets of Chemical Carcinogens

• Chemical carcinogens react with DNA at guanine residues (most commonly)
• This produces adducts (covalent carcinogen-DNA bonds)
• If misrepaired, this leads to permanent mutations - particularly in proto-oncogenes (e.g., RAS) and tumor suppressor genes (e.g., TP53)
• RAS mutations are found in many chemically-induced cancers; the specific mutation depends on which carcinogen was used

Metabolism and Susceptibility

• Cytochrome P-450 enzymes (CYP1A1, CYP1A2) convert procarcinogens to ultimate carcinogens
• Genetic polymorphisms in these enzymes explain why some individuals are more susceptible to chemical carcinogenesis
• Detoxification enzymes (glutathione S-transferase) can inactivate carcinogens; low activity of these enzymes increases cancer risk

Q4. Short Note: Cardiac Edema

Definition

Pathogenesis

• In congestive heart failure (CHF), reduced cardiac output leads to pooling of blood in the venous circulation
• This raises capillary hydrostatic pressure in the venous end of capillaries
• The outward force (hydrostatic pressure) exceeds the inward force (plasma oncotic pressure) → fluid shifts into the interstitium

• Left heart failure: Back-pressure transmitted to pulmonary veins → pulmonary edema (dyspnea, orthopnea, paroxysmal nocturnal dyspnea, pink frothy sputum; seen as "bat-wing" opacity on chest X-ray)
• Right heart failure: Back-pressure transmitted to systemic veins → dependent edema (pitting edema of ankles and feet, sacral edema in bedridden patients), ascites, hepatomegaly

• Renin → Angiotensin I → Angiotensin II (via ACE)
• Angiotensin II → stimulates aldosterone release → Na+ and water retention in the kidneys
• Increased blood volume → raises venous hydrostatic pressure further → worsens edema
• Angiotensin II also stimulates ADH (vasopressin) release → water retention
• This vicious cycle perpetuates and worsens edema

Features of Cardiac Edema

Morphological Features

• Grossly: heavy, wet, boggy lungs
• Microscopically: alveolar spaces filled with pink fluid; "heart failure cells" - alveolar macrophages laden with hemosiderin (from phagocytosed RBCs that leaked from congested capillaries)

• Grossly: mottled appearance - centrilobular dark red areas surrounded by paler periportal zones, resembling a cut nutmeg
• Microscopically: centrilobular hepatic necrosis with hemorrhage and scattered inflammatory cells

Clinical Correlation

• Investigations: Chest X-ray (cardiomegaly + bat-wing pulmonary edema), serum BNP/NT-proBNP (elevated in heart failure), echocardiography (reduced ejection fraction), serum albumin (normal, unlike nephrotic edema)
• Treatment targets: Diuretics (furosemide) reduce fluid overload; ACE inhibitors/ARBs block RAAS; beta-blockers; digoxin