Acinus	The functional unit of the lung: a respiratory bronchiole and its alveolar ducts and alveolar sacs. (Note: The definition of "acinus" varies somewhat in the literature)
Angiocentric	A descriptive term for lesions that appear to center on vessels. Angiocentricity may be appreciated with lesions that are actually centered on vessels (either arteries or veins) or with lesions that are distributed around the vessels and which may or may not secondarily infiltrate the vessels. "Angiocentric" should be distinguished from "angiotropic", which describes intravascular lymphomatosis (angiotropic lymphoma) as well as any lesion prone to intraluminal invasion of a vessel
Asbestos body	A ferruginous body with an asbestos (thin translucent) core
Bronchiolitis obliterans	A histologic term referring to 2 broad groups of lesions: proliferative bronchiolitis obliterans with granulation tissue polyps filling small airways, and fibrotic bronchiolitis obliterans with permanent scarring and stenosis or obliteration of airways. Proliferative bronchiolitis obliterans is often associated with organizing pneumonia in which the same reparative reaction that is present in the bronchiole extends out into more distal parenchyma. Histologic bronchiolitis obliterans may or may not be associated with clinical evidence of airway obstruction
Capillaritis	Mural inflammation (vasculitis) of the capillaries analogous to leukocytoclastic vasculitis at other sites
Cellular bronchiolitis	Cellular infiltrates, either acute or chronic, that involve bronchioles and that are usually mural, although a luminal acute inflammatory exudate is frequently present
Diffuse alveolar damage	(DAD) A nonspecific acute response to lung injury, primarily affecting alveoli and alveolar walls. DAD includes both acute injury and repair phases. DAD is the usual histologic correlate of the ARDS
Exudate	Cells, cellular debris, and/or fibrin (with or without edema fluid) within airspaces
Ferruginous body	Particulate material covered by hemosiderin, highlighted by iron (Prussian blue) stains. A subset of ferruginous bodies are asbestos bodies, which usually have a beaded surface, club-shaped ends, and a central thin translucent core; the last feature is diagnostic of an asbestos body
Follicular bronchitis/ bronchiolitis	Lymphoid hyperplasia with germinal center formation along airways
Honeycombing	An end-stage lesion seen with many pulmonary injuries, usually chronic interstitial pneumonias. The lung architecture is permanently reorganized into functionally useless tissue
Hyaline membranes	Dense eosinophilic membranes seen in a number of acute lung diseases, usually DAD. Hyaline membranes are most prominent in and along the surfaces of alveolar ducts. Hyaline membranes are composed of fibrin, cell debris, hemorrhage, and proteinous material
Interstitial fibrosis	Interstitial thickening resulting from mature collagen deposition (as contrasted to edematous new connective tissue) in the interstitium and organization in airspaces
Interstitial infiltrate	An infiltrate of cells, regardless of type, in the interstitium. Usually the cells are mononuclear; however, interstitial infiltrates of neutrophils and eosinophils are also recognized
Interstitium	Includes alveolar walls, interlobular septa, and connective tissue around bronchovascular structures
Lambertosis	Bronchiolar epithelial metaplasia occurring in peribronchiolar regions of scarred bronchioles, presumably extending through Lambert's canals
Lobule	The pulmonary lobule (sometimes called the secondary lobule) refers to an anatomic unit of lung parenchyma, 0.50 to 2.00 cm in diameter, bounded by interlobular septa. Lobules can be appreciated on cut section of lung tissue and on the pleural surface, and they usually comprise some ten to 10-30 acini
Lymphatic distribution	A low-power distribution appreciated in a number of diffuse lung diseases in which the pathologic changes are seen along the lymphatic routes: in the pleura, in the interlobular septa, and along bronchovascular bundles. The lymphatic vessels themselves are often not appreciated as such
Lymphoid hyperplasia	Lymphoid follicles containing germinal centers, usually found in a lymphatic distribution
Miliary distribution	Lesions (usually small nodules) appear randomly scattered throughout the lung. Sometimes they may appear perivenular in distribution. Miliary nodules are characteristic of hematogenous granulomatous infections and some viral infections
Obstructive pneumonia	Changes seen in the lung parenchyma distal to an obstruction, regardless of cause. This disorder typically includes accumulation of foamy macrophages within alveoli, inspissated mucus, prominent Type II alveolar lining cells, mononuclear interstitial infiltrates, and, in some cases, organizing pneumonia
Old granuloma	A term for a nodule with fibrotic rim and necrotic center that is usually, but not always, the result of an old healed infectious granuloma. A palisaded histiocytic rim around the necrosis may be present
Organizing pneumonia	A reparative reaction in the lung with organizing granulation tissue filling alveolar ducts and associated alveoli with only focal attachment to the interstitium where the proliferation originates. Proliferative bronchiolitis obliterans often accompanies this reaction. This type of reaction is associated with modest interstitial infiltrates and prominent Type II alveolar lining cells, and it should be distinguished from interstitial fibrosis
Peribronchiolar	A distribution of pathologic changes around small airways including the wall of bronchioles and the immediately adjacent alveoli
Septal	Septal refers to the interlobular septa and structures in them, including veins, lymphatics, and connective tissue.