PA25.1-7 | Respiratory System — Glossary

Bacteria with a waxy mycolic acid cell wall that resist decolorisation by acid-alcohol in the Ziehl-Neelsen stain, appearing red against a blue background; mycobacteria are the archetypal AFB.

A bacterium whose cell wall contains mycolic acids that resist decolorisation with acid-alcohol in the Ziehl-Neelsen stain; M. tuberculosis and M. leprae are the most clinically important examples.

Non-invasive lung adenocarcinoma showing pure lepidic growth along alveolar walls (≤3 cm); formerly called bronchioloalveolar carcinoma (BAC); excellent prognosis after resection.

Radiological sign (seen on erect CXR or CT) of a cavity containing liquid below and gas above; characteristic of lung abscess that has communicated with the bronchial tree.

Exaggerated bronchoconstrictor response to nonspecific stimuli (cold air, methacholine); the physiological hallmark of asthma.

Structural changes in the asthmatic airway from repeated inflammation, including sub-epithelial fibrosis, smooth muscle hypertrophy, and goblet cell metaplasia, causing partially irreversible obstruction.

EML4-ALK fusion oncogene in ~5% of lung adenocarcinomas; targetable with crizotinib and alectinib; mutually exclusive with EGFR mutation.

A serine protease inhibitor produced by the liver that normally inhibits neutrophil elastase in the lung; its deficiency leads to unopposed alveolar wall destruction.

Accumulation of neutrophils, fibrin, and proteinaceous fluid within alveolar airspaces, filling but not destroying alveolar architecture; the microscopic hallmark of acute bacterial pneumonia.

Needle-shaped asbestos fibres (crocidolite/blue asbestos; amosite/brown asbestos) that are biopersistent in lung tissue and carry the highest mesothelioma risk.

Inert deposition of carbon (coal) pigment in lung macrophages and regional lymph nodes; causes no functional impairment and is seen in all urban dwellers and smokers.

Black carbon particles phagocytosed by alveolar macrophages, deposited in peribronchiolar tissue and lymph nodes; the microscopic basis of coal-worker's pneumoconiosis and urban lung blackening.

Asbestos fibre coated with a haemosiderin-glycoprotein complex, appearing as a golden-brown, dumbbell-shaped or beaded rod on H&E; a marker of asbestos exposure but not alone diagnostic of asbestosis.

Pneumonia caused by inhalation of oropharyngeal or gastric contents, predominantly involving anaerobic organisms; typically affects the right lower lobe or posterior segment of the right upper lobe.

Type I IgE-mediated hypersensitivity asthma triggered by environmental allergens; associated with elevated serum IgE, eczema, and allergic rhinitis.

Pneumonia caused by organisms lacking a peptidoglycan cell wall (Mycoplasma) or obligate intracellular pathogens; produces an interstitial rather than lobar pattern and does not respond to beta-lactam antibiotics.

Encrustation of vessel walls by basophilic DNA material liberated from necrotic small-cell carcinoma cells; considered pathognomonic of SCLC.

Clinical phenotype of predominant chronic bronchitis: stocky, cyanosed, hypercapnic, oedematous patient with pronounced V/Q mismatch from mucus plugging.

Permanent, abnormal dilatation of bronchi and bronchioles due to destruction of the bronchial wall's muscular and elastic components, caused by obstruction and/or recurrent infection.

Bacterial pneumonia characterised by multiple patchy foci of suppurative consolidation centred on bronchioles, typically bilateral and lower-lobe predominant.

Emphysematous blebs greater than 1 cm in diameter, often subpleural, representing confluent destroyed airspaces; may rupture causing spontaneous pneumothorax.

Calcium-binding protein expressed by mesothelial cells and neurons; positive in mesothelioma, negative in metastatic adenocarcinoma — used as a key IHC marker to distinguish the two.

Rapid development of large necrobiotic nodules in the lungs of pneumoconiosis patients (especially coal miners) who also have rheumatoid arthritis, reflecting immune-mediated pathogenesis.

Organisation of unresolved fibrinous exudate into granulation tissue and ultimately collagenous scar, producing a firm, fleshy, airless lung segment (failure of resolution).

A specific type of granuloma with central caseous necrosis surrounded by epithelioid macrophages, Langhans giant cells, and a lymphocytic cuff; pathognomonic of tuberculosis and a few other infections (histoplasma, coccidioides).

A form of coagulative necrosis (with liquefactive elements) in which the tissue is converted into homogeneous, granular, cheese-like acellular material with complete loss of normal architecture; produced by the destructive arm of Type IV hypersensitivity in TB.

Emphysema affecting the proximal acinus (respiratory bronchioles), predominantly in the upper lobes; strongly associated with cigarette smoking.

Bipyramidal, needle-shaped crystals composed of galectin-10 (lysophospholipase) from eosinophil membranes; found in asthmatic sputum and tissues.

Clinically defined as productive cough for at least 3 consecutive months in at least 2 consecutive years, after exclusion of other causes.

Airless solidification of lung tissue due to replacement of air in alveoli by exudate, cells, or fluid; on gross examination the lung loses its spongy texture and resembles liver (hepatisation) or solid tissue.

Right ventricular hypertrophy and/or dilation caused by pulmonary hypertension arising from a primary lung or pulmonary vascular disease.

Smearing and streaming of fragile tumour cell nuclei produced by biopsy forceps pressure; a diagnostically useful feature in SCLC reflecting the mechanical fragility of neuroendocrine cells.

Whorled mucus plugs cast from small airways, shed into sputum in asthma; a microscopic hallmark of mucus hypersecretion.

Acute pattern of lung injury characterised by hyaline membrane formation, interstitial oedema, and type II pneumocyte proliferation; the substrate for acute respiratory distress syndrome.

Activating mutation (most commonly exon 19 deletion or L858R point mutation) in the epidermal growth factor receptor gene; found in ~40–50% of Asian lung adenocarcinomas; confers sensitivity to tyrosine kinase inhibitors.

Peripheral rim calcification of hilar lymph nodes on chest X-ray, virtually pathognomonic of silicosis, caused by calcium deposition in silica-laden lymph node capsules.

Permanent abnormal enlargement of airspaces distal to the terminal bronchiole, with alveolar wall destruction and without fibrosis.

Accumulation of pus (frank purulent exudate) within the pleural cavity, most commonly as a complication of bacterial pneumonia or lung abscess.

An activated macrophage that has undergone morphological transformation into an elongated cell with pale, abundant cytoplasm and an oval vesicular nucleus, resembling an epithelial cell; the core element of the granuloma.

The most common (~60%) histological subtype of malignant mesothelioma; shows tubulo-papillary architecture; best prognosis of the three subtypes.

A form of hypersensitivity pneumonitis caused by inhalation of thermophilic actinomycetes (Saccharopolyspora rectivirgula) from mouldy hay; reversible if antigen exposure is stopped early.

An asbestos fibre coated with haemosiderin and protein, giving a golden-brown, dumbbell-shaped or beaded rod appearance on H&E; synonymous with asbestos body in lung tissue; their presence confirms significant asbestos exposure.

The fraction of forced vital capacity expelled in the first second; the primary spirometric marker distinguishing obstructive (<0.70) from restrictive (>=0.70) lung disease.

Cluster of actively proliferating fibroblasts in a myxoid stroma at the advancing edge of fibrosis in UIP/IPF; represents the active progression front and is the histological hallmark of IPF.

Failure of a macrophage to completely engulf a particle or organism that is too large, resulting in continued release of lysosomal enzymes and fibrogenic cytokines into the extracellular space.

A rapid molecular diagnostic test using cartridge-based nucleic acid amplification to simultaneously detect M. tuberculosis DNA and rifampicin resistance within 2 hours; first-line diagnostic under India's NTEP.

The combination of the Ghon focus (parenchymal) and caseating ipsilateral hilar/mediastinal lymph nodes connected by a zone of lymphangitis; represents the primary complex of tuberculosis (Ranke complex).

The initial parenchymal lesion of primary TB — a small (1–2 cm) subpleural area of caseous consolidation in the lower part of the upper lobe or upper part of the lower lobe; usually heals and calcifies.

Global Initiative for Chronic Obstructive Lung Disease severity classification based on post-bronchodilator FEV1 % predicted: Stage I (>80%), II (50-79%), III (30-49%), IV (<30%).

Stage 3 of lobar pneumonia; the lobe becomes grey-brown as red blood cells lyse and macrophages replace neutrophils; marks the onset of resolution.

Conversion of aerated lung tissue to a firm, liver-like consistency due to alveolar flooding with fibrin, cells, and exudate in lobar pneumonia.

End-stage lung destruction pattern in which cystic spaces lined by bronchiolar-type epithelium replace normal alveoli, visible on HRCT as sub-pleural clustered cysts; seen in advanced IPF and other chronic ILDs.

Triad of ipsilateral ptosis, miosis, and anhidrosis caused by interruption of the sympathetic supply to the eye; in lung cancer, caused by Pancoast tumour invading the stellate ganglion.

Immunologically mediated interstitial lung disease caused by repeated inhalation of organic antigens (e.g., mould spores, avian proteins), combining Type III immune complex and Type IV cell-mediated reactions.

Paraneoplastic syndrome of periosteal proliferation, clubbing, and joint pain associated predominantly with NSCLC; mechanism involves VEGF and neurogenic pathways.

Visible desmosomes between adjacent squamous carcinoma cells seen on H&E at high magnification, indicating squamous differentiation; one of the two hallmarks of SCC (with keratin pearls).

Pneumonia in which inflammation is confined predominantly to the alveolar walls and septa rather than the alveolar spaces; caused by atypical organisms (Mycoplasma, viruses).

The triad of bronchiectasis, sinusitis, and situs inversus caused by primary ciliary dyskinesia; defective nodal cilia during embryogenesis fail to determine left-right asymmetry.

Concentric whorls of squamous cells with central keratinisation, pathognomonic of squamous cell carcinoma in histological sections.

Neuroendocrine (APUD) cells in the bronchial epithelium that are the cell of origin for SCLC and pulmonary carcinoid tumours.

Paraneoplastic autoimmune disorder caused by anti-voltage-gated calcium channel (VGCC) antibodies in SCLC; presents with proximal weakness that paradoxically improves with repeated activity.

A multinucleate giant cell formed by fusion of epithelioid macrophages in granulomatous inflammation; characterised by nuclei arranged in a horseshoe or peripheral arc pattern, distinguishing it from foreign-body giant cells (random nuclear distribution).

A state of persistent immune response to M. tuberculosis antigens without evidence of active disease; bacilli are viable but dormant within granulomas; Mantoux/IGRA positive, no symptoms, chest X-ray normal or showing only calcified Ghon lesion.

A Gram-negative, intracellular bacterium causing Legionnaires' disease (severe atypical CAP); acquired via inhalation of contaminated aerosols; survives inside macrophages by blocking phagosome–lysosome fusion.

Growth of adenocarcinoma cells along pre-existing alveolar walls without stromal invasion; the non-invasive precursor pattern (adenocarcinoma in situ).

Bacterial pneumonia in which an entire lobe becomes uniformly consolidated, classically proceeding through four pathological stages; most commonly caused by Streptococcus pneumoniae.

A localised area of suppurative (liquefactive) necrosis within lung parenchyma, forming a pus-filled cavity often with an air-fluid level on imaging.

Primary malignancy of mesothelial cells lining the pleura; strongly linked to amphibole asbestos exposure with a 25–45 year latency; forms a thick pleural rind that encases the lung.

Multidrug-resistant tuberculosis — defined as resistance to at least isoniazid AND rifampicin, the two most important first-line anti-TB drugs; requires second-line drug regimens for 18–20 months.

Malignant tumour of the mesothelial lining of the pleura (or peritoneum), strongly associated with asbestos exposure; has a latency of 30–40 years and no safe threshold of exposure.

A pattern of widespread haematogenous dissemination of TB producing multiple 1–2 mm caseating granulomas (resembling millet seeds) in the lungs and other organs; results from erosion of a caseous lesion into a blood vessel.

The obligate aerobic, acid-fast bacillus responsible for tuberculosis; its mycolic acid-rich cell wall confers acid-fastness and blocks phagolysosome fusion, enabling intracellular survival.

Long-chain fatty acids unique to the cell wall of mycobacteria; responsible for acid-fastness, resistance to killing, and stimulation of the host granulomatous response.

Intracellular multiprotein complex activated by silica and other crystalline particles; triggers caspase-1 activation and IL-1β processing, amplifying macrophage-driven inflammation in silicosis.

Well-formed aggregate of epithelioid macrophages and multinucleate giant cells without central necrosis (caseation), characteristic of sarcoidosis and distinguishing it from TB.

Umbrella term for lung carcinomas other than SCLC, comprising squamous cell carcinoma, adenocarcinoma, and large cell carcinoma; potentially resectable when localised.

Hospital-acquired pneumonia developing >48 hours after admission; typically caused by Gram-negative bacilli (Pseudomonas, Klebsiella) or MRSA, reflecting the hospital flora.

A histological feature in which closely packed tumour cell nuclei conform to each other's shapes, resulting in irregular angular nuclear outlines; characteristic of SCLC and indicative of high nuclear-to-cytoplasmic ratio.

Descriptive term for the small cells of SCLC — dark, oval nuclei with scant cytoplasm and nuclear moulding, approximately twice the size of a lymphocyte.

The self-perpetuating pathogenic loop in bronchiectasis: airway obstruction -> secretion retention -> bacterial infection -> wall destruction -> permanent dilatation -> further obstruction.

Emphysema involving the entire acinus uniformly, with lower-lobe predominance; the hallmark of alpha-1-antitrypsin deficiency.

Lung carcinoma (usually SCC or adenocarcinoma) arising at the lung apex (superior sulcus); invades lower brachial plexus (arm pain) and cervical sympathetic chain (Horner syndrome).

Signs and symptoms caused by tumour secretion of hormones, peptides, or immune cross-reactivity, occurring at sites remote from the tumour itself.

Clinical phenotype of predominant emphysema: thin, barrel-chested, markedly dyspnoeic but relatively non-cyanosed patient who hyperventilates to maintain oxygenation.

Discrete ivory-white acellular hyaline collagen lesion on the parietal pleura, the most common manifestation of asbestos exposure; not pre-malignant.

The gross appearance of mesothelioma as a thick, white, rubbery sheet of tumour encasing and compressing the lung, growing along pleural surfaces rather than forming a single discrete mass.

A grating sound on auscultation produced by inflamed visceral and parietal pleural surfaces rubbing against each other; heard in fibrinous pleuritis complicating lobar pneumonia.

Non-neoplastic reaction of the lung to inhaled mineral or inorganic dust, leading to progressive pulmonary fibrosis; examples include CWP, silicosis, and asbestosis.

A cholesterol-dependent cytolysin secreted by Streptococcus pneumoniae that disrupts alveolar epithelial and endothelial cell membranes, facilitating invasion and alveolar flooding.

Infection-driven inflammation of the lung parenchyma involving alveoli and/or terminal airways, regardless of the causative organism.

Tuberculous spondylitis — haematogenous seeding of vertebral bodies (typically L1–L4) causing progressive vertebral destruction, collapse, and angular kyphosis (gibbus deformity); the most common form of skeletal TB.

An autosomal recessive disorder of ciliary structure (usually dynein arm defects) causing impaired mucociliary clearance, recurrent sinopulmonary infections, and situs inversus when all three features present (Kartagener syndrome).

Advanced stage of coal worker's pneumoconiosis characterised by large (>2 cm) confluent black fibrotic masses in the upper lobes, causing severe restrictive disease and cor pulmonale.

A severe form of pneumoconiosis in which silicotic or coal nodules coalesce into large fibrous masses >2 cm in upper lobes, severely restricting lung function and predisposing to secondary TB.

A key COPD mechanism in which smoking-activated neutrophils release elastase that overwhelms alpha-1-AT inhibition, causing alveolar destruction.

Peptide secreted ectopically by squamous cell carcinoma of the lung; binds PTH receptors to cause humoral hypercalcaemia of malignancy without bone metastases.

Simultaneous presence of pus and air in the pleural space, typically resulting from rupture of a lung abscess that communicates with both the pleura and a bronchus.

A pseudoaneurysm of a pulmonary arterial branch within the wall of a TB cavity, formed by inflammatory weakening of the vessel wall; rupture causes massive haemoptysis and is a life-threatening emergency.

Stage 2 of lobar pneumonia; the lobe is dark red and firm due to alveoli packed with fibrin, neutrophils, and red blood cells.

The ratio of mucous gland thickness to bronchial wall thickness (epithelium to cartilage); >0.5 is diagnostic of chronic bronchitis (normal <=0.4).

Particles in the 1–5 µm size range that bypass upper airway defences and reach alveoli, where they are engulfed by macrophages and trigger fibrogenic reactions.

Pulmonary disorder characterised by reduced lung compliance leading to decreased lung volumes (TLC, FVC) with a normal or elevated FEV1/FVC ratio, caused by stiff lung parenchyma or chest wall restriction.

Dark reddish-brown sputum characteristic of pneumococcal lobar pneumonia, caused by haemoglobin degradation products (methaemoglobin, biliverdin) in the alveolar exudate.

Systemic amyloid deposition (serum amyloid A protein) in response to chronic inflammatory or suppurative disease; a late complication of bronchiectasis and chronic osteomyelitis.

Deposition of AA (serum amyloid A-derived) protein in organs as a complication of chronic inflammatory or suppurative conditions such as lung abscess, osteomyelitis, or bronchiectasis.

Amyloid deposition caused by chronic infection/inflammation (including TB) with sustained high levels of serum amyloid A protein; amyloid deposits primarily in kidney, liver, and spleen, causing nephrotic syndrome and organomegaly.

TB arising in a previously sensitised individual, typically from reactivation of latent bacilli; characteristically involves the pulmonary apices, causes cavitation, and has minimal lymph node involvement.

Infected thrombus fragments originating from a remote source (e.g., right-sided infective endocarditis, septic thrombophlebitis) that lodge in pulmonary vasculature and cause multiple peripheral lung abscesses.

Hyponatraemia caused by ectopic ADH secretion from SCLC; characterised by hyponatraemia with inappropriately concentrated urine despite plasma hypo-osmolality.

The characteristic lesion of silicosis: concentrically arranged (whorled) layers of hyalinised collagen containing birefringent silica particles, with a peripheral cellular zone of macrophages and lymphocytes.

Co-occurrence of silicosis and active tuberculosis, occurring because silica-laden alveolar macrophages have impaired bactericidal capacity and cannot effectively kill engulfed Mycobacterium tuberculosis.

High-grade neuroendocrine carcinoma of the lung arising from Kulchitsky cells; characterised by oat-cell morphology, early dissemination, and marked paraneoplastic potential; treated with chemotherapy, not surgery.

Liquefactive necrosis mediated by neutrophil-derived proteolytic enzymes; the dominant pattern in bacterial lung abscess, resulting in a cavity containing semi-liquid pus.

Obstruction of the superior vena cava by a central lung tumour (most commonly SCLC) causing facial oedema, plethora, and distended neck and arm veins.

International tumour staging system (IASLC 8th edition for lung); T = tumour size/invasion, N = nodal status, M = metastasis; determines surgical resectability and prognosis.

A nuclear transcription factor expressed in lung adenocarcinoma (and thyroid carcinoma); its presence on IHC supports adenocarcinoma lineage and helps exclude mesothelioma and squamous cell carcinoma.

A severe pattern of TB in which bacilli drain from an upper-lobe cavity into the bronchial tree and spread to dependent zones, causing confluent caseous consolidation in the lower lobes; highly infectious and life-threatening.

Delayed-type (cell-mediated) hypersensitivity mediated by sensitised CD4+ Th1 lymphocytes and activated macrophages, peaking 48–72 hours after antigen exposure; the mechanism of caseous necrosis and Mantoux positivity in TB.

The histological pattern of idiopathic pulmonary fibrosis, characterised by temporal heterogeneity (areas of normal lung adjacent to fibrosis), fibroblastic foci at the advancing edge, and sub-pleural basal honeycombing.

Extensively drug-resistant tuberculosis — MDR-TB with additional resistance to any fluoroquinolone and at least one injectable second-line agent (amikacin, kanamycin, or capreomycin); associated with very poor treatment outcomes.

A differential stain for acid-fast organisms using carbol fuchsin (red dye fixed with heat), decolorised with acid-alcohol, and counterstained with methylene blue; AFB appear as beaded red rods on a blue background.