Discussion

Pneumocystis pneumonia (PCP) is a serious opportunistic infection caused by the ubiquitous airborne fungus Pneumocystis jirovecii, primarily affecting people with impaired immunity, including those with HIV—especially when CD4 counts fall below 200 cells/mm³—as well as patients with hematologic malignancies, autoimmune disease, primary immunodeficiencies, renal transplants, or immunosuppressive therapy. PCP typically involves the lungs, where the host inflammatory response damages the alveolar–capillary interface; however, rare extrapulmonary involvement may occur. Symptoms generally develop over days to weeks and include fever, progressive shortness of breath, dry cough, chest pain, fatigue, chills, and low oxygen saturation.

Pneumocystis jirovecii has a small asexual trophic form (1–4 µm), which predominates during infection, and a larger, sexually reproductive cystic form (7–8 µm). In H&E-stained tissue sections, alveolar spaces are filled with eosinophilic, foamy, amorphous material composed of proliferating fungi and cell debris, typically accompanied by mild, plasma cell–rich interstitial inflammation. The Gomori methenamine silver (GMS) stain highlights the organisms as thin-walled, uniformly sized, round, oval, crescentic, or boat-shaped forms, sometimes resembling “crushed ping-pong balls,” with nuclei often positioned near the cell wall.

In fulminant disease, diffuse alveolar damage and organizing pneumonia may develop. A granulomatous response is an uncommon manifestation, occurring in approximately 5% of cases and most often affecting non-HIV immunocompromised patients or those with partial recovery of cell-mediated immunity. Concurrent infections, such as cytomegalovirus, may coexist.

References

1. Crothers J, Russell-Goldman E, Milner DA. Pneumocystosis. In: Diagnostic Pathology: Infectious Diseases, 3rd  ed.: Elsevier; 2024.

2. Tripathy A, Husain AN. Pneumocystis. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/lungnontumorpcp.html. Accessed October 10th, 2026.