General pathology: cell injury, inflammation and neoplasia

In short

General pathology is the study of how cells and tissues respond to injury — adapting, dying, inflaming, healing, or escaping control and becoming a tumour. It is the bridging subject of Phase 2 MBBS: almost every clinical diagnosis later rests on these mechanisms, and almost every biopsy report uses this vocabulary. It is examined in university Paper I of Pathology, in NEET-PG/INI-CET (consistently one of the highest-yield areas), and in viva on gross and histopathology specimens. On the job it underpins histopathology reporting, cytology, tumour boards, and medico-legal certification of cause of death.

Explained

Cellular adaptation and the limits of homeostasis

• Hypertrophy — increase in cell size, so organ size, without cell division. Physiological (skeletal muscle in a weightlifter, uterus in pregnancy) or pathological (left ventricle in hypertension/aortic stenosis).
• Hyperplasia — increase in cell number; only in cells capable of division. Hormonal (breast at puberty), compensatory (liver regeneration after partial hepatectomy), pathological (endometrial hyperplasia, benign prostatic hyperplasia).
• Atrophy — reduced cell size and function from disuse, denervation, ischaemia, loss of endocrine stimulus, ageing or malnutrition. Mediated by the ubiquitin–proteasome pathway and autophagy.
• Metaplasia — reversible replacement of one differentiated cell type by another better suited to stress: squamous metaplasia of bronchial epithelium in smokers; intestinal (columnar) metaplasia of lower oesophagus in Barrett's; squamous metaplasia in bladder/ cervix.
• Dysplasia — disordered growth with loss of uniformity and architectural orientation; not adaptation but a pre-malignant change. Nuclear pleomorphism, hyperchromasia, raised N:C ratio, mitoses above the basal layer. Dysplasia involving the full thickness without crossing the basement membrane = carcinoma in situ.
• Key exam line: metaplasia is reversible; dysplasia may be reversible if the stimulus is removed; carcinoma in situ is not.

Cell injury: causes, mechanisms, reversible versus irreversible

• Causes: hypoxia/ischaemia (commonest), physical agents, chemicals and drugs, infectious agents, immunological reactions, genetic defects, nutritional imbalance, ageing.
• Hypoxia vs ischaemia: hypoxia is oxygen lack (anaemia, high altitude, CO poisoning); ischaemia is loss of blood supply, so it also starves the tissue of glucose and substrate — therefore injures faster and more severely.
• Four principal intracellular targets: (1) mitochondrial ATP generation, (2) membrane integrity, (3) protein synthesis and folding, (4) DNA and the cytoskeleton.
• Sequence in ischaemia: ↓oxidative phosphorylation → ↓ATP → failure of Na⁺/K⁺-ATPase → cellular swelling (hydropic change), ER swelling, blebs → anaerobic glycolysis → ↓pH, clumping of nuclear chromatin → detachment of ribosomes, ↓protein synthesis → (if hypoxia continues) mitochondrial and lysosomal membrane damage → irreversible injury.
• Points of no return: (a) inability to reverse mitochondrial dysfunction, (b) profound membrane disturbance. Morphological hallmarks of irreversibility: severe mitochondrial swelling with amorphous densities, extensive membrane defects, lysosomal rupture.
• Reversible changes: cellular swelling, fatty change (steatosis), blebbing, myelin figures, ER dilatation.
• Free radical (oxidative) injury: superoxide, H₂O₂, hydroxyl radical, peroxynitrite. Generated in normal respiration, inflammation (NADPH oxidase), radiation, redox-active drugs, transition metals (Fenton reaction), nitric oxide. Damage by lipid peroxidation, protein cross-linking/oxidation, and DNA strand breaks.
• Antioxidant defences: superoxide dismutase, catalase, glutathione peroxidase; vitamins A, C, E; glutathione; transport proteins (transferrin, ferritin, ceruloplasmin) that sequester iron and copper.
• Reperfusion injury: restoring blood flow to ischaemic tissue worsens damage through a burst of reactive oxygen species, calcium overload, neutrophil influx and complement activation — relevant to myocardial infarction after thrombolysis/PCI.
• Chemical injury: direct (mercuric chloride binding sulfhydryl groups; cyanide inhibiting cytochrome oxidase) or via toxic metabolites (CCl₄ → CCl₃• by cytochrome P-450; paracetamol → NAPQI, treated with N-acetylcysteine).
• Intracellular accumulations: fat (liver in alcohol, diabetes, malnutrition), protein (Russell bodies, Mallory–Denk hyaline), glycogen, pigments — exogenous (carbon/anthracosis, tattoo) and endogenous (lipofuscin "wear-and-tear", melanin, haemosiderin, bilirubin).
• Pathological calcification: dystrophic — in dead/damaged tissue with normal serum calcium (atheroma, calcified valves, caseous TB nodes, Mönckeberg's); metastatic — in normal tissue with hypercalcaemia (hyperparathyroidism, vitamin D excess, bone metastases, milk-alkali, sarcoidosis).

Cell death: necrosis, apoptosis and related pathways

• Necrosis — cell death with loss of membrane integrity, enzymatic digestion, leakage of contents and consequent inflammation; always pathological.
• Patterns of necrosis:
  • *Coagulative* — architecture preserved, protein denaturation dominant; all solid organs infarcted except brain.
  • *Liquefactive* — digestion dominant; brain infarct, pyogenic abscess.
  • *Caseous* — cheesy, amorphous granular debris with loss of architecture; tuberculosis (a high-yield point for India's TB burden).
  • *Fat necrosis* — enzymatic (acute pancreatitis, with saponification) or traumatic (breast).
  • *Fibrinoid* — immune complexes plus fibrin in vessel walls; vasculitis, malignant hypertension, rheumatic carditis.
  • *Gangrenous* — not a distinct mechanism: dry (coagulative, ischaemic limb) versus wet (superadded bacterial liquefaction); gas gangrene with *Clostridium perfringens*.
• Apoptosis — regulated, energy-dependent death of single cells; cell shrinkage, chromatin condensation, apoptotic bodies, no inflammation, phosphatidylserine flip aids phagocytosis.
  • *Intrinsic (mitochondrial) pathway:* BH3-only sensors → BAX/BAK pore → cytochrome c release → caspase-9/apoptosome; BCL-2 and BCL-XL are anti-apoptotic.
  • *Extrinsic (death receptor) pathway:* FAS–FAS-L, TNFR1 → FADD → caspase-8 → executioner caspase-3/7.
  • *Physiological:* embryogenesis, endometrial shedding, involution, deletion of autoreactive lymphocytes. *Pathological:* DNA damage, misfolded protein accumulation, viral infection, CD8⁺ T-cell killing (also uses granzyme B/perforin).
• Other pathways: necroptosis (RIPK1/RIPK3–MLKL; "programmed necrosis"), pyroptosis (inflammasome, caspase-1, IL-1β release — pathogen-driven), ferroptosis (iron-dependent lipid peroxidation), autophagy (nutrient deprivation; can protect or kill).
• Autolysis vs necrosis — autolysis is post-mortem self-digestion without inflammatory reaction; a recurring viva and forensic question.

Acute and chronic inflammation

• Definition: a protective vascular and cellular response of vascularised tissue to injury, delivering leucocytes and plasma proteins to the site. Cardinal signs (Celsus/Virchow): rubor, tumor, calor, dolor, functio laesa.
• Vascular events: transient vasoconstriction → vasodilatation (heat, redness) → increased permeability (exudation of protein-rich fluid) → stasis with haemoconcentration → margination of leucocytes.
• Mechanisms of increased permeability: endothelial contraction/gap formation (histamine, bradykinin — immediate transient), endothelial injury (direct, immediate sustained), leucocyte-mediated injury, transcytosis (VEGF), delayed prolonged leakage (sunburn).
• Exudate vs transudate: exudate = high protein, specific gravity > 1.020, high LDH, cellular, from increased permeability; transudate = low protein, SG < 1.012, from altered hydrostatic/oncotic pressure (cardiac failure, nephrotic syndrome, cirrhosis). Light's criteria separate them in pleural fluid.
• Cellular events: margination → rolling (selectins — E, P, L; ligands sialyl-Lewis X) → firm adhesion (integrins LFA-1/Mac-1 with ICAM-1; VLA-4 with VCAM-1) → transmigration (PECAM-1/CD31) → chemotaxis → recognition → phagocytosis (engulfment, phagolysosome formation, killing by NADPH-oxidase respiratory burst → H₂O₂ → myeloperoxidase + Cl⁻ → HOCl, plus iNOS and granule enzymes) → resolution.
• Cell timing: neutrophils dominate the first 6–24 h, monocytes/macrophages the next 24–48 h; exceptions — *Pseudomonas* (persistent neutrophils), viral infection (lymphocytes), allergy/parasites (eosinophils).
• Defects: LAD-1 (β2-integrin CD18), LAD-2 (fucosyl transferase/sialyl-Lewis X), chronic granulomatous disease (NADPH oxidase; nitroblue tetrazolium/DHR flow test), Chediak–Higashi (LYST, giant granules), MPO deficiency.
• Chemical mediators — know source and action:
  • *Vasoactive amines:* histamine (mast cells, basophils, platelets) — vasodilatation, venular permeability; serotonin (platelets).
  • *Arachidonic acid metabolites:* cyclo-oxygenase → prostaglandins (PGE₂ pain and fever; PGI₂ vasodilator/antiaggregatory; TXA₂ vasoconstrictor/aggregatory); lipoxygenase → LTB₄ (chemotaxis), LTC₄/D₄/E₄ (bronchospasm, permeability); lipoxins are anti-inflammatory.
  • *Cytokines:* TNF and IL-1 (endothelial activation, acute-phase response, fever, cachexia), IL-6 (CRP, hepcidin), chemokines (IL-8/CXCL8 — neutrophil chemotaxis), IFN-γ (macrophage activation).
  • *Complement:* C3a/C5a anaphylatoxins; C5a chemotactic and activating; C3b opsonin; C5–C9 membrane attack complex.
  • *Kinins:* bradykinin — pain, permeability, vasodilatation. *Clotting:* fibrinopeptides, thrombin.
  • *Others:* PAF, nitric oxide, neuropeptides (substance P), lysosomal enzymes, ROS.
• Outcomes of acute inflammation: complete resolution, healing by fibrosis/scarring, abscess formation, or progression to chronic inflammation.
• Chronic inflammation: persistent inflammation with mononuclear infiltrate (macrophages, lymphocytes, plasma cells), tissue destruction and simultaneous repair (angiogenesis, fibrosis). Causes — persistent infection (TB, leprosy, syphilis, fungi), hypersensitivity/autoimmunity (RA, IBD), prolonged exposure to toxic agents (silica → silicosis, lipids → atheroma).
• Macrophage polarisation: M1 (classically activated by IFN-γ, LPS) — microbicidal, ROS/NO, IL-1, IL-12, IL-23; M2 (alternatively activated by IL-4, IL-13) — repair, fibrosis, IL-10, TGF-β.
• Granulomatous inflammation: focal collection of activated macrophages (epithelioid cells) with a lymphocyte rim, often multinucleate Langhans giant cells, formed by Th1/IFN-γ-driven type IV hypersensitivity. *Caseating* — tuberculosis (confirm with Ziehl–Neelsen, culture, or NAAT such as CBNAAT/Truenat under the National TB Elimination Programme); *non-caseating* — sarcoidosis, Crohn's, leprosy (tuberculoid), berylliosis, foreign body, fungal; also schistosomiasis and cat-scratch (suppurative).
• Systemic effects: fever (PGE₂ via hypothalamus), leucocytosis with left shift, acute-phase proteins (CRP, fibrinogen, SAA, hepcidin), raised ESR, and in severe cases SIRS/sepsis with DIC and shock.

Healing, repair and fibrosis

• Labile cells (surface epithelium, marrow) regenerate continuously; stable cells (liver, kidney, pancreas, fibroblasts, smooth muscle) divide when stimulated; permanent cells (neurons, cardiac and skeletal muscle) cannot replace losses — hence scarring after infarction.
• Phases: haemostasis and inflammation → proliferation (angiogenesis, fibroblast migration, granulation tissue of new capillaries, oedematous ECM, fibroblasts) → maturation and remodelling (type III collagen replaced by type I; MMPs and TIMPs; wound contraction by myofibroblasts).
• Healing by primary union (clean surgical incision, apposed edges, minimal granulation, fine scar) versus secondary union (large open wound, abundant granulation tissue, marked contraction, larger scar).
• Tensile strength: ~10% at 1 week; ~70–80% of normal by 3 months, never fully restored.
• Key growth factors: VEGF (angiogenesis), PDGF, FGF-2, EGF, TGF-β (the dominant fibrogenic cytokine — stimulates collagen, inhibits MMPs).
• Factors delaying healing: infection (commonest), poor blood supply, diabetes, protein/vitamin C (scurvy — defective hydroxylation of proline/lysine) and zinc deficiency, glucocorticoids (anti-TGF-β), foreign body, movement, old age, irradiation.
• Aberrations: deficient scar/dehiscence, ulceration, hypertrophic scar (within wound limits), keloid (beyond wound margins; commoner in darker skin, frequent in Indian practice), exuberant granulation, contracture (burns), desmoid/fibromatosis.
• Fracture healing: haematoma → procallus/soft callus (fibrocartilage) → bony (woven bone) callus → remodelling to lamellar bone.
• Amyloidosis (grouped with cell injury in most syllabi): extracellular deposition of misfolded protein in β-pleated sheet, Congo red positive with apple-green birefringence under polarised light. AL (plasma cell dyscrasia), AA (SAA; secondary to chronic inflammation — in India classically TB, bronchiectasis, rheumatoid arthritis, leprosy), Aβ₂M (dialysis), ATTR (familial/senile), Aβ (Alzheimer), AANF, calcitonin (medullary thyroid carcinoma). Presents with nephrotic syndrome, restrictive cardiomyopathy, hepatosplenomegaly, macroglossia; biopsy of abdominal fat pad or rectum confirms.

Neoplasia: nomenclature, biology and carcinogenesis

• Neoplasm: an abnormal mass of tissue whose growth is excessive, autonomous and persists after the initiating stimulus is removed (Willis). All tumours have parenchyma (neoplastic cells) and stroma (connective tissue, vessels — desmoplasia if abundant).
• Nomenclature: benign mesenchymal = cell of origin + *-oma* (lipoma, leiomyoma); malignant mesenchymal = sarcoma; malignant epithelial = carcinoma (adenocarcinoma if glandular, squamous cell carcinoma if squamous). Misnomers: lymphoma, melanoma, seminoma, mesothelioma, hepatoma, glioma — all malignant.
• Mixed tumours (pleomorphic adenoma of salivary gland), teratoma (all three germ layers; mature/immature), hamartoma (disorganised native tissue), choristoma (ectopic normal tissue).
• Benign vs malignant: differentiation and anaplasia, rate of growth and mitoses, local invasiveness (capsule vs infiltration), and metastasis — the single most reliable feature of malignancy (exceptions: gliomas and basal cell carcinoma are locally aggressive but rarely metastasise).
• Anaplasia features: pleomorphism, hyperchromatic nuclei, high N:C ratio (toward 1:1), prominent nucleoli, abnormal/atypical mitoses, loss of polarity, tumour giant cells, ischaemic necrosis.
• Hallmarks of cancer: self-sufficiency in growth signals; insensitivity to growth inhibition; evasion of apoptosis; limitless replicative potential (telomerase); sustained angiogenesis; invasion and metastasis; reprogrammed metabolism (Warburg effect — aerobic glycolysis, the basis of FDG-PET); immune evasion; genomic instability; tumour-promoting inflammation.
• Oncogenes (gain of function, dominant): RAS (point mutation — commonest in human cancers), MYC (translocation t(8;14) in Burkitt), ABL (t(9;22) BCR-ABL in CML — target of imatinib), HER2/ERBB2 (amplification in breast cancer — trastuzumab), EGFR (lung adenocarcinoma — gefitinib/osimertinib), BRAF V600E, ALK, CYCLIN D1/CDK4.
• Tumour suppressor genes (loss of function, usually need both alleles — Knudson's two-hit): RB (retinoblastoma, the governor of G1/S), TP53 ("guardian of the genome"; mutated in >50% of cancers; Li–Fraumeni), APC (familial adenomatous polyposis, colorectal), BRCA1/2 (breast–ovarian; PARP-inhibitor sensitive), NF1/NF2, VHL, WT1, PTEN, CDKN2A, TGF-β pathway, mismatch repair genes (MLH1, MSH2 → Lynch syndrome, microsatellite instability).
• Carcinogenic agents: chemical initiators (direct-acting alkylating agents; indirect procarcinogens — polycyclic aromatic hydrocarbons in tobacco smoke, aflatoxin B₁ → TP53 codon 249, aromatic amines, nitrosamines, asbestos, vinyl chloride, arsenic) and promoters (hormones, phorbol esters — promotion is non-mutagenic and reversible); radiation (UV-B → pyrimidine dimers; ionising radiation → thyroid, leukaemia); oncogenic viruses — HPV 16/18 (cervix: E6 degrades p53, E7 inactivates RB), EBV (nasopharyngeal carcinoma, Burkitt, Hodgkin), HBV/HCV (hepatocellular carcinoma), HTLV-1, KSHV/HHV-8; bacteria — *H. pylori* (gastric carcinoma and MALT lymphoma).
• Metastasis routes: lymphatic (carcinomas, usually to the first draining node), haematogenous (sarcomas; liver and lung are commonest sites because of portal and caval drainage), transcoelomic/seeding (ovarian carcinoma, Krukenberg tumour from stomach), perineural, and iatrogenic implantation.
• Metastatic cascade: loss of E-cadherin and epithelial–mesenchymal transition → degradation of basement membrane by MMPs/cathepsins → migration → intravasation → survival in circulation → arrest and extravasation → colonisation in a permissive "seed and soil" microenvironment (chemokine receptors such as CXCR4 explain organ tropism).
• Grading reflects differentiation (microscopic); staging reflects extent (anatomical, more prognostic). TNM (AJCC/UICC): T0–T4 primary size/extent, N0–N3 regional nodes, M0/M1 distant metastasis, then stage group 0–IV.
• Tumour effects: cachexia (TNF, IL-1, IL-6, proteolysis-inducing factor — not simple starvation), pain, obstruction, haemorrhage, infection; paraneoplastic syndromes — SIADH (small cell lung cancer), ectopic ACTH (Cushing), PTH-related protein (squamous cell lung carcinoma → hypercalcaemia), hypoglycaemia (insulin-like factors), Lambert–Eaton, acanthosis nigricans, hypertrophic osteoarthropathy, migratory thrombophlebitis (Trousseau).
• Laboratory diagnosis: FNAC, exfoliative cytology (Pap smear, the basis of India's cervical screening under the NP-NCD programme), frozen section, histopathology, immunohistochemistry (cytokeratin — epithelial; vimentin — mesenchymal; LCA/CD45 — lymphoid; S-100/HMB-45/Melan-A — melanoma; ER/PR/HER2 — breast; Ki-67 proliferation index), flow cytometry, FISH, PCR and next-generation sequencing. Serum markers: AFP (hepatocellular, germ cell), β-hCG (choriocarcinoma, germ cell), CEA (colorectal), CA-125 (ovary), CA-19.9 (pancreas), PSA (prostate), calcitonin (medullary thyroid), LDH (lymphoma burden). Markers are for monitoring and risk, never for diagnosis alone.
• Indian context and regulation: tobacco (smoked and smokeless) drives oral, lung and oesophageal cancer — regulated by COTPA, 2003 and the National Tobacco Control Programme; cervical and breast cancer screening sit in the National Programme for Prevention and Control of Non-Communicable Diseases; cancer is a notifiable condition in several states and data flow to the National Cancer Registry Programme (ICMR–NCDIR); radiotherapy and nuclear-medicine sources are regulated by AERB; antineoplastic use and biosafe disposal follow the Bio-Medical Waste Management Rules, 2016 (as amended) — cytotoxic waste in yellow category. Sex determination for prenatal diagnosis is restricted under the PCPNDT Act, 1994; HPV vaccination is being introduced in the Universal Immunisation Programme in a phased manner — check the current national schedule, as roll-out details change.

Key points

• Hypertrophy vs hyperplasia - bigger cells vs more cells; both may coexist (pregnant uterus).
• Metaplasia - one differentiated cell type replaced by another, reversible; dysplasia - disordered pre-malignant growth.
• Ischaemia worse than hypoxia - loses oxygen plus glucose and substrate delivery.
• ATP depletion - the central event; Na⁺/K⁺ pump failure causes the earliest reversible change, cellular swelling.
• Irreversibility - marked by mitochondrial dysfunction with amorphous densities and severe membrane damage.
• Necrosis vs apoptosis - necrosis: swelling, membrane rupture, inflammation, groups of cells; apoptosis: shrinkage, intact membrane, no inflammation, single cells, caspase-dependent.
• Caseous necrosis - hallmark of tuberculosis; liquefactive necrosis - brain infarct and abscess.
• Dystrophic vs metastatic calcification - dead tissue with normal calcium vs normal tissue with hypercalcaemia.
• Vascular triad of acute inflammation - vasodilatation, increased permeability, stasis with leucocyte margination.
• Leucocyte adhesion cascade - selectins (rolling) → integrins (adhesion) → PECAM-1 (transmigration) → chemotaxis.
• C5a, LTB₄, IL-8, bacterial products - the four classic chemotactic agents.
• Histamine, bradykinin, PGE₂ - permeability/vasodilatation and pain; TXA₂ vs PGI₂ are opposing platelet/vessel mediators.
• Granuloma - epithelioid macrophages ± Langhans giant cells; caseating = TB, non-caseating = sarcoidosis/Crohn's/leprosy.
• Granulation tissue - new capillaries + fibroblasts + myofibroblasts; TGF-β is the main fibrogenic cytokine.
• Permanent cells (neurons, cardiac muscle) heal only by scar — the reason infarcts fibrose.
• Metastasis - the only absolutely reliable evidence of malignancy; E-cadherin loss starts the cascade.
• Two-hit hypothesis - both alleles of a tumour suppressor (RB, TP53, APC, BRCA) must be lost.
• Grade vs stage - grade is differentiation under the microscope; stage (TNM) is anatomical extent and predicts prognosis better.

Formulas and examples

Example 1 — Tumour growth and why cancers present late. A single transformed cell (10⁻⁹ g) has a doubling time of 60 days. The smallest clinically detectable mass is about 1 g (≈10⁹ cells).

• Doublings needed = log₂(10⁹) = 9 ÷ log₁₀2 = 9 ÷ 0.30103 ≈ 29.9 ≈ 30 doublings.
• Time = 30 × 60 = 1,800 days ≈ 4.9 years of silent growth.
• Only 10 further doublings (1 g → ~1 kg ≈ 10¹² cells) are usually fatal — about 600 days more.
• Interpretation: roughly three-quarters of the tumour's life is pre-clinical, which is exactly why screening (Pap smear, mammography, oral examination) and not symptom-waiting detects cancer early.

Example 2 — Ki-67 and grading a breast carcinoma. IHC shows 184 positive nuclei out of 800 counted.

• Ki-67 = (184 ÷ 800) × 100 = 23% — high proliferative activity.
• Histology: no tubule formation (score 3), marked pleomorphism (score 3), 14 mitoses/10 HPF (score 3 in the Nottingham system). Total = 3 + 3 + 3 = 9 → Grade 3, poorly differentiated.
• Tumour 2.8 cm (T2), 2 of 14 axillary nodes positive (N1), no distant spread (M0) → Stage IIB. Grade 3 with high Ki-67 and ER/PR/HER2 all negative = triple-negative, chemotherapy-dependent.

Example 3 — Pleural fluid: exudate or transudate? Pleural protein 4.6 g/dL, serum protein 7.0 g/dL; pleural LDH 420 U/L, serum LDH 300 U/L (upper normal 250 U/L).

• Protein ratio = 4.6 ÷ 7.0 = 0.657 (> 0.5 ✓)
• LDH ratio = 420 ÷ 300 = 1.40 (> 0.6 ✓)
• Pleural LDH vs 2/3 × 250 = 166.7 U/L → 420 > 166.7 ✓
• All three satisfied → exudate. With lymphocyte predominance and raised ADA in an Indian adult, tuberculous pleural effusion is the first consideration; confirm with pleural fluid NAAT or biopsy showing caseating granuloma.

Example 4 — Cardiac enzyme release as a measure of irreversible injury. A patient reaches hospital 5 hours after chest pain onset. Troponin I is 8.4 ng/mL (normal < 0.04 ng/mL) = 210 times the upper limit.

• Troponin leaks only after sarcolemmal rupture, i.e. irreversible injury — reversible ischaemia (angina) gives no rise.
• Timeline to recall: myocyte necrosis begins ~20–30 minutes of total ischaemia; coagulative necrosis is visible microscopically by 4–12 h; neutrophils peak 1–3 days; macrophages and granulation tissue 3–10 days; dense collagenous scar by 2 months (cardiac myocytes are permanent cells, so no regeneration).

Where it is used

• Histopathology and cytopathology laboratories (AIIMS, PGIMER, Tata Memorial, SRL, Metropolis, Dr Lal PathLabs): reporting biopsies, FNACs, Pap smears, frozen sections; grading and TNM staging drive treatment.
• Clinical practice: interpreting CRP/ESR and total counts, choosing anti-inflammatory therapy, managing diabetic foot and burn wounds (healing principles), and counselling after an infarct.
• Oncology and tumour boards: IHC panels, molecular markers and targeted therapy selection (HER2–trastuzumab, BCR-ABL–imatinib, EGFR/ALK inhibitors, PD-L1 for immunotherapy).
• Public health and registries: NCRP/ICMR–NCDIR cancer registries, tobacco-control work under COTPA 2003, cervical and oral cancer screening under the NCD programme.
• Diagnostics industry and research: antibody and kit development, biomarker validation, CDSCO-regulated IVD approval, clinical trials under New Drugs and Clinical Trials Rules, 2019; ICMR/CSIR and pharma discovery roles.
• Forensic and medico-legal work: distinguishing ante-mortem injury (vital reaction, inflammation) from post-mortem change (autolysis) and certifying cause of death.

Interview questions

• Q: What is the single earliest reversible morphological change in cell injury and why? A: Cellular (hydropic) swelling, because ATP depletion disables the Na⁺/K⁺-ATPase so sodium and water enter the cell.
• Q: How would you distinguish necrosis from apoptosis on a slide? A: Necrosis shows swollen cells in groups with ruptured membranes and an inflammatory infiltrate; apoptosis shows single shrunken cells with condensed chromatin, apoptotic bodies and no inflammation.
• Q: Why is reperfusion sometimes more damaging than the ischaemia itself? A: Returning oxygen generates a burst of reactive oxygen species and brings in neutrophils and complement, while calcium overload finishes off cells that were still borderline viable.
• Q: Which type of necrosis occurs in the brain and why is it different? A: Liquefactive — the brain is rich in lipid and hydrolytic enzymes and poor in supporting stroma, so digestion outweighs protein denaturation.
• Q: What differentiates an exudate from a transudate at the bedside? A: Exudate is protein-rich and cellular from increased vascular permeability (infection, malignancy); transudate is protein-poor from hydrostatic or oncotic imbalance (heart failure, nephrotic syndrome) — Light's criteria confirm it.
• Q: Name the chemotactic mediators for neutrophils. A: C5a, leukotriene B₄, IL-8 (CXCL8) and bacterial products such as N-formyl-methionyl peptides.
• Q: What makes a granuloma form, and how do you tell a tuberculous one from sarcoid? A: Persistent, poorly degradable antigen driving a Th1/IFN-γ type IV reaction; tuberculous granulomas caseate and may show acid-fast bacilli, sarcoid granulomas are non-caseating and tightly packed.
• Q: Why does a myocardial infarct leave a scar while the liver regenerates? A: Cardiac myocytes are permanent cells that cannot divide, whereas hepatocytes are stable cells capable of compensatory hyperplasia.
• Q: Difference between a keloid and a hypertrophic scar? A: A hypertrophic scar stays within the original wound boundary and may regress; a keloid grows beyond the wound margins, recurs after excision and is commoner in pigmented skin.
• Q: Which single feature confirms malignancy, and what are the exceptions? A: Metastasis; the exceptions are tumours that are malignant yet rarely metastasise, such as gliomas and basal cell carcinoma.
• Q: Grade versus stage — which matters more? A: Grade describes differentiation microscopically; stage describes anatomical extent by TNM and is generally the stronger prognostic determinant and the basis of treatment choice.
• Q: How do HPV E6 and E7 cause cervical cancer? A: E6 targets p53 for degradation and E7 binds and inactivates RB, so the G1/S checkpoint and apoptotic response are both lost in high-risk HPV 16/18 infection.
• Q: Can a tumour marker be used to diagnose cancer? A: No — markers such as CEA, CA-125 and PSA lack specificity and sensitivity; they are used for risk stratification, monitoring treatment response and detecting recurrence, with diagnosis resting on histopathology.

Common mistakes

• Calling all programmed cell death "apoptosis" — necroptosis, pyroptosis and ferroptosis are regulated yet morphologically necrotic.
• Writing that gangrene is a separate mechanism of necrosis; it is coagulative (dry) or liquefactive-infected (wet) necrosis of a limb or bowel.
• Swapping dystrophic and metastatic calcification — the discriminator is the serum calcium, not the site.
• Describing dysplasia as irreversible or as cancer; and forgetting that an intact basement membrane distinguishes carcinoma in situ from invasive carcinoma.
• Listing mediator names without source and action — examiners ask "which cell, which effect", and confusing TXA₂ with PGI₂ loses the mark.
• Assuming "-oma" always means benign (lymphoma, melanoma, seminoma are malignant) and that a capsule proves benignity.
• Confusing grading with staging, or quoting mitoses per 10 HPF without noting that current WHO systems use mitoses per mm².
• Reporting a tumour marker as diagnostic, or omitting the denominator when quoting Ki-67 or mitotic counts.

Related topics

Systemic pathology · Haematology · Immunology and ELISA · Anticancer drugs and immunopharmacology · Medical microbiology: infections and diagnosis · Forensic medicine: injuries, autopsy and medico-legal practice