Histopathology of Pneumonia

Lobar Pneumonia

Lobar pneumonia occurs when entire lobe(s) become acutely inflamed, leading to the alveoli filling with fluid, cell debris, inflammatory cells, and fibrin (consolidation). Both grossly and under the microscope, the affected lobe(s) are clearly distinct compared to surrounding normal lobes and will be visibly discoloured, lose their spongy appearance, and will appear consolidated.

^{Presented by Dr. Jennifer Kong using DHPLC specimen A0201}

Histologically, the alveolar walls will remain intact and thin, with stained material filling the lumen. The inflammation will affect the entire lobe(s) uniformly, with complete or nearly complete consolidation of the alveoli. As with other inflammatory processes, the blood vessels become congested. There are four main stages of lobar pneumonia that can be identified depending on what types of materials or cells can be found within the alveolar fluids:

^{Presented by Noah Stewart using a histology slide of H&E stained lung with lobar pneumonia  (DHPLC e-slide: PATH 425-001) under a CC-BY-NC license}

Bronchopneumonia

Bronchopneumonia occurs when the infection begins within the bronchi/bronchioles, and then descends into the nearby alveoli. Because of this, consolidation of the alveoli occurs in patches around the affected bronchi/bronchioles. This pattern may or may not be localized to a single lobe of the lung.

^{Presented by Noah Stewart using a histology slide of H&E stained lung with bronchopneumonia  (University of Michigan virtual Slide Box 151127)}

Interstitial Pneumonia

Unlike lobar or bronchopneumonia, interstitial pneumonia is characterized by the inflammation and congestion of the alveolar walls/interstitial tissue. Interstitial pneumonia can be identified by diffuse, patchy areas of inflamed interstitial tissue. The alveolar lumens will be mostly normal. Within the inflamed areas, the alveolar walls will be congested and may appear thicker than the alveolar lumens as they become filled with fluid, inflammatory cells, debris, and possibly scar tissue.

^{Presented by Dr. Jennifer Kong using DHPLC specimen A0204. All rights reserved}

^{Presented by Noah Stewart using a histology slide of H&E stained lung with interstitial pneumonia  (DHPLC e-slide: PATH 425-014) under a CC-BY-NC license}

Outcomes of Pneumonia

A patient may present with different signs and symptoms, based on the type of pneumonia affecting them. Lobar pneumonia and bronchopneumonia normally present as typical pneumonia. This includes malaise, fever/chills, productive cough with purulent sputum (pus), difficulty breathing (dyspnea) or an increased rate of breathing (tachypnea). Interstitial pneumonia often presents as atypical pneumonia, which has a slow onset, malaise, fever without chills, muscle soreness, headaches, nonproductive (dry) cough.

Pneumonia has a wide range of outcomes depending on patient factors, the specific pathogen, the severity of infection, and the length of infection, among others. Severe complications, such as the formation of lung abscesses can occur in rare cases. An abscess is a cavity that contains liquefied necrotic (dead) tissue, pus, and microorganisms.

^{Presented by Noah Stewart using a histology slide of H&E stained lung with abscess  (DHPLC e-slide: PATH 425-003) under a CC-BY-NC license}

Review Questions

Normal lung histology

Pneumonia histopathology

Section Summary

• Lobar pneumonia presents as a typical pneumonia and affects one or more lobes of the lung uniformly. Inflammation makes blood vessels more prominent while consolidation contributes to hepatization of the affected lobe(s).
• Bronchopneumonia presents as a typical pneumonia, originating in the larger airways and radiating to nearby alveoli. Consolidation is patchy and may be diffuse throughout the lung, but may progress to the more uniformly distributed lobar pneumonia.
• Interstitial pneumonia presents as an atypical pneumonia. Consolidation is absent but interstitial tissue may become inflammed and thicken and/or scar.