RESUMEN
Pneumocystis pneumonia (PCP) is caused by the yeast-like fungus Pneumocystis jirovecii, which is specific to humans. PCP could be a source of opportunistic infection in adults that are immunosuppressed and children with prematurity or malnutrition. The diagnosis should be confirmed by identification of the causative organism, by analysis of the sputum, a bronchoalveolar lavage or a tissue biopsy. In both histologic and cytologic specimens, the cysts are contained within frothy exudates, which form aggregated clumps. The cysts often collapse forming crescent-shaped bodies that resemble ping-pong balls. We recently diagnosed nine cases of PCP using an immunohistochemical stain for Pneumocystis. The patients consisted of five human immunodeficiency virus positive individuals, two renal transplant recipients, and two patients with a malignant disease. All nine patients were infected with P. jirovecii, which was positive for monoclonal antibody 3F6. In conclusion, the immunohistochemical stain used in this report is a new technique for the detection of P. jirovecii infection.