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Pneumocystis jerovecii is a eukaryotic microorganism that is the human-specific counterpart of Pneumocystis carinii (which is found in a wide range of animals). Previously thought to be protozoan, it is now classified as a yeast-like fungus. In humans, it represents an important opportunistic pulmonary infection with significant morbidity and mortality in immunosuppressed patients. Morphologically, the affected lung tissue typically shows characteristic intra-alveolar foamy exudate with interstitial lymphoplasmacytic infiltrate and occasional intra-alveolar fibrous bodies. GMS stain is often helpful and quite effective in highlighting cyst forms of the organisms.
See Related Content for references:
1) An T, Tabaczka P. Arch Pathol Lab Med. 2004;128(3):363-364.
2) Wazir JF, Ansari NA. Arch Pathol Lab Med. 2004;128(9):1023-7.