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Reviewers: Elliot Weisenberg, M.D. (see Reviewers page)
Revised: 30 August 2011, last major update August 2011
Copyright: (c) 2003-2011, PathologyOutlines.com, Inc.


● Pneumonia is defined as any infection of lung parenchyma
● Lung is #1 site for infections that cause lost workdays
● Infection of lung is more frequent than any other visceral organ
● Generally microorganisms are inhaled, but pneumonia may also occur through hematogenous spread or direct inoculation


● Pneumonia is due to impairment of normal defense mechanisms or lowered host resistance
● Normal defense mechanisms are nasal clearance (sneezing, blowing, swallowing), tracheobronchial clearance (mucociliary action), and alveolar clearance (alveolar macrophages)
● Impairment is due to primary or acquired immunosuppression, suppression of cough reflex (drugs, virus, coma, anesthesia), injury to mucociliary apparatus (smoking, virus, Kartegeners syndrome), injury to macrophages (tobacco, alcohol, anoxia), pulmonary congestion/edema, or accumulation of secretions (cystic fibrosis)
● Note: viral pneumonia predisposes to bacterial pneumonia

Clinical features

● Often divided into community acquired (Minerva Anestesiol 2011;77:196), hospital acquired (Clin Infect Dis 2010;51 Suppl 1:S42) and aspiration pneumonia
● Common agents: Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Legionella pneumophila, Pseudomonas aeruginosa, coliforms
● Complications: abscess, empyema, organization, sepsis, meningitis
● Consolidation: exudative solidification of lung
● Signs and symptoms of pneumonia: shortness of breath, fever, productive cough, malaise, friction rub (if fibrinous pleuritis)
● Bronchopneumonia: patchy consolidation of lung centered on bronchi; may progress to lobar pneumonia; patterns of bronco- and lobar pneumonia may overlap
● Lobar pneumonia: affects entire lung, but now rare due to antibiotics; associated with increased virulence of organism or increased host vulnerability (infants, elderly); may be due to extension of existing bronchiolitis or bronchitis

Gross images

Bronchopneumonia - various images

Lobar pneumonia - various images

Micro description

● Bronchopneumonia: neutrophils in bronchi, bronchioles and adjacent alveolar spaces; lipid pneumonia, if marked, has lipid laden macrophages
● Lobar pneumonia: initially congestion with bacteria and few neutrophils; then red hepatization (grossly resembles liver) with massive congestion, neutrophils, fibrin; then gray hepatization with fibrinopurulent exudate and organization; then resolution with resorption of exudate

Micro images

Aspiration pneumonia

Bronchopneumonia - various images

Virtual slides

Early pneumonia

Aspiration pneumonia-adult

Aspiration pneumonia-infant

Organizing pneumonia


Bronchopneumonia with abscess

Lobar pneumonia

End of Lung-nontumor > Infections > General/pneumonia

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