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Lobar Pneumonia

Pneumonia lobaris

For medical students2 min readUpdated 2026-10-10

Lobar pneumonia is an acute infectious-allergic disease of the lungs involving the visceral pleura and characterized by the formation of a fibrinous exudate. The pathological process typically involves an entire lobe of the lung.

Primary pathogenStreptococcus pneumoniae (over 90% of cases)

Stages of the disease4 classic sequential stages

Mortality rateApproximately 3% with antibiotic therapy

Pathogenetic mechanismType I hypersensitivity reaction

Etiology and Pathogenesis

The primary causative agent is the pneumococcus (Streptococcus pneumoniae), accounting for over 90% of cases. The disease can also be caused by Klebsiella pneumoniae (Friedländer's bacillus) and other microorganisms. Transmission occurs via airborne droplets.

The pathogenesis is based on a type I hypersensitivity reaction within the alveoli and alveolar ducts. The early phases are explained by two main theories:

Aspiration theory — microorganisms from the upper respiratory tract enter the alveoli under predisposing factors (e.g., hypothermia), triggering a hyperergic reaction.
Hematogenous-immune complex theory — the pathogen enters the bloodstream and parenchyma via the lymphoid tissue of the nasopharynx, followed by secondary hematogenous seeding, damaging the microvasculature.

Stages of Morphological Changes

The classical progression of the pathological process includes 4 stages:

Stage of congestion (inflammatory edema) — first 24 hours. Characterized by marked capillary hyperemia, interstitial edema, and a fluid exudate containing bacteria, which rapidly spreads via the pores of Kohn.
Stage of red hepatization — day 2. The lobe becomes firm and airless, resembling liver tissue. The alveolar lumina are packed with erythrocytes and fibrin networks.
Stage of gray hepatization — days 4–6. The dark lung tissue fills with massive numbers of polymorphonuclear leukocytes and macrophages phagocytosing the bacteria.
Stage of resolution — begins on days 9–11. Enzymatic lysis of fibrin by leukocyte proteases occurs, and the exudate is cleared via lymphatic drainage and expectoration.

Complications and Outcomes

Complications of lobar pneumonia are divided into pulmonary and extrapulmonary. They develop against the background of impaired immunity and protective cell depletion.

Pulmonary complications: carnification of the lung (organization of the exudate with connective tissue replacement due to lymphostasis), acute lung abscess, gangrene, empyema, and life-threatening acute respiratory distress syndrome (ARDS).
Extrapulmonary complications: associated with bacteremia (occurring in 30% of cases). Lymphogenous spread can lead to purulent mediastinitis or pericarditis, while hematogenous spread can cause purulent meningitis, metastatic brain abscesses, or infective endocarditis involving the tricuspid valve.

Outcome: in most cases favorable with complete recovery, although pleural adhesions may persist. Death results from acute cardiorespiratory failure or purulent complications.

Mnemonic

The four stages of lobar pneumonia can be easily memorized by their physical state: Edema (congestion) $\rightarrow$ Blood (red hepatization) $\rightarrow$ Gray leukocytes (gray hepatization) $\rightarrow$ Absorption (resolution).

Frequently asked questions

What macroscopic changes are observed in the lung during the stage of gray hepatization?

During the stage of gray hepatization, the affected lung lobe macroscopically increases in size, becomes heavy, firm, and completely airless. On cross-section, the lung tissue surface acquires a characteristic gray color and a granular appearance. The visceral pleura is thickened, appears cloudy, and is covered with prominent fibrinous deposits.

How does lobar pneumonia differ from bronchopneumonia?

Lobar pneumonia differs from bronchopneumonia in the extent of involvement and the nature of the pathological process.

Feature	Lobar Pneumonia	Bronchopneumonia
Extent of involvement	Lobar pneumonia: involves an entire lobe of the lung	Patchy, focal distribution
Pleural involvement	Visceral pleura is typically involved; hence the synonym "pleuropneumonia"	Not typically a primary feature
Spread	Involves respiratory compartments with intra-alveolar exudation	Spreads bronchogenically
Status	An independent nosological entity	A secondary, focal form of pneumonia

What microscopic features characterize pulmonary carnification?

Pulmonary carnification is characterized microscopically by the replacement of intra-alveolar exudate with connective tissue. During this process, the following changes are observed:

Alveolar lumina — instead of fibrinous masses and cellular exudate, maturing and subsequently mature fibrous connective tissue is found.
Alveolar septa — thickened and sclerosed.
Acinar architecture — disrupted, with air spaces markedly reduced or absent.

Why is lobar pneumonia also called lobar pneumonia or pleuropneumonia?

The term "lobar" reflects the morphology of the process, which affects an entire lung lobe. The term "pleuropneumonia" is used because the visceral pleura is invariably involved in the inflammation.

What is pulmonary carnification and why does it occur?

Carnification is a pulmonary complication where the fibrinous exudate is invaded and replaced by granulation and connective tissue (organization). This occurs due to impaired lymphatic drainage and lymphostasis.

Which pneumococcal serotypes are most virulent?

Serotypes 1, 2, 5, 7, 12, and 14 exhibit the highest virulence and are capable of infecting initially healthy individuals.

Go deeper

Pneumococcal serotyping and virulence characteristics in adults and children
Comparison of aspiration and hematogenous-immune complex pathogenesis theories
Cytological composition of exudate across different stages of inflammation
Modern pathomorphosis of lobar pneumonia and abortive forms
Role of bacterial enzymes (hyaluronidase, leukocidin) in the development of disease staging