P ulmonary
Aliya N. Husain, M.D.
VII. Infections
A. Acute bacterial pneumonia.
  1. Culture and sensitivity results, extent of involvement and host resistance are much more important than histologic changes.
  2. All bacterial pneumonias are characterized by presence of neutrophils in air spaces and small bronchioles.

  3. 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.

  4. Bronchopneumonia is patchy involving only small bronchioles and adjacent alveoli as compared to lobar pneumonia which involves one or more lobes.
B. Atypical pneumonia
  1. Most viral and mycoplasmal pneumonias cause interstitial accumulation of lymphocytes.

  2. CMV (cytomegalovirus) can be identified easily since it causes enlargement of infected cells which contain single larger intranuclear and multiple small intracytoplasmic inclusions.

  3. Identification of other viruses usually depends on serologic studies and cultures.
C. Actinomycosis and nocardiosis
  1. These are filamentous bacteria which can be recognized on tissue sections.

  2. 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.

  3. Nocardia is a branching filamentous, gram-positive bacterium which is also weakly acid-fact. Inflammation is also neutrophilic.
D. Tuberculosis
  1. 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.

  2. Acid-fast bacilli can sometimes be demonstrated by the Zeihl-Neelson stain on tissue sections. Cultures are much more sensitive.

  3. 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.

  1. Candidiasis: Pseudohyphae and budding yeasts.

  2. Aspergillosis: Septate, true hypae with acute angle branding.

  3. Mucormycosis: Non-septate, true hyphae, broad, ribbon-like, with wide angle branching.

  4. Histoplasmosis: Granulomatous inflammation caused by small budding yeasts (3-5 microns). Grossly, tree-barking appearance of granuloma is characteristic.

  5. Blastomycosis: Mixed acute and granulomatous inflammation caused by large budding yeasts, thick-walled with broad-based bud.

  6. Coccidioidomycosis: Granulomatous inflammation caused by variable sized daughter cysts within large cyst wall.

  7. 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:
  1. 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.