36-year-old male HIV positive male succumbs to disseminated Kaposi sarcoma (specimen from autopsy).
- The architecture of the lung parenchyma is preserved.
- Pulmonary septae are thickened displaying a lymphohistiocytic infiltrate and congested capillaries.
- In alveoli accumulation of a variable number of macrophages and enlarged pneumocytes. The latter display big nuclei and contain nuclear and cytoplasmic intensely stained inclusion bodies (virus particles).
The brown staining obtained in positive immunohistochemistry reactions has to be distinguished from: (1) anthracotic pigment (coal dust), a coarse black pigment, located in macrophages around bronchovascular bundles, in interlobular septa and beneath the pleura; (2) hemosiderin pigment which is golden yellow to brown and granular (3) formalin pigment which is a brown, granular, birefringent deposit seen both intracellularly and extracellularly in tissues fixed in formalin solutions.