|A. Acute bacterial pneumonia.|
- Culture and sensitivity results, extent of involvement and host resistance are much more important than histologic changes.
- All bacterial pneumonias are characterized by presence of neutrophils in air spaces and small bronchioles.
- Grossly, in lobar pneumonia the lung appears red and consolidated (red hepatization) followed by grey-white appearance (grey hepatization) due to lysis of RBC's , followed by either complete resolution or fibrosis.
- Bronchopneumonia is patchy involving only small bronchioles and adjacent alveoli as compared to lobar pneumonia which involves one or more lobes.
|B. Atypical pneumonia|
- Most viral and mycoplasmal pneumonias cause interstitial accumulation of lymphocytes.
- CMV (cytomegalovirus) can be identified easily since it causes enlargement of infected cells which contain single larger intranuclear and multiple small intracytoplasmic inclusions.
- Identification of other viruses usually depends on serologic studies and cultures.
|C. Actinomycosis and nocardiosis|
- These are filamentous bacteria which can be recognized on tissue sections.
- In actinomycosis there are sulphur granules which are ball-shaped bacterial colonies, gram-positive, with club shaped ends of radially oriented filaments. Adjacent lung tissue has neutrophilic inflammation.
- Nocardia is a branching filamentous, gram-positive bacterium which is also weakly acid-fact. Inflammation is also neutrophilic.
- The histologic hallmark is caseating granulomata with Langhan's type giant cells. The granuloma is a rounded collection of macrophages and lymphocytes containing multinucleated giant cells, the nuclei of which are arranged at the periphery in a horse-shoe shape.
- Acid-fast bacilli can sometimes be demonstrated by the Zeihl-Neelson stain on tissue sections. Cultures are much more sensitive.
- Secondary TB has a much high incidence of large areas of caseating necrosis. Otherwise, primary and secondary TB are histologically similar.
|E. Fungal infections: ||All of the following are GMS and PAS positive.|
- Candidiasis: Pseudohyphae and budding yeasts.
- Aspergillosis: Septate, true hypae with acute angle branding.
- Mucormycosis: Non-septate, true hyphae, broad, ribbon-like, with wide angle branching.
- Histoplasmosis: Granulomatous inflammation caused by small budding yeasts (3-5 microns). Grossly, tree-barking appearance of granuloma is characteristic.
- Blastomycosis: Mixed acute and granulomatous inflammation caused by large budding yeasts, thick-walled with broad-based bud.
- Coccidioidomycosis: Granulomatous inflammation caused by variable sized daughter cysts within large cyst wall.
- Cryptococcosis: Minimal or no inflammation, budding yeasts with mucin positive capsule (which looks like a halo on H&E) and thin-based buds.
|F. Pneumocystis pneumonia:|
- Typical histologic picture shows intraalveolar, acellular, frothy material with minimal inflammation. Silver stain (GMS) shows 7-8 micron, non-budding cysts which may be cup-shaped or helmet-shaped. Some have central black dot.