Pneumocystis jirovecii is a fungal organism that causes pneumonia primarily in immunocompromised hosts. The organism adheres to type I pneumocytes via fibronectin and mannose receptors, proliferating within the alveolar space. As trophic forms and cysts accumulate, they create a characteristic foamy, eosinophilic exudate that fills alveoli and blocks gas exchange. Impaired cell-mediated immunity (CD4 < 200 cells/mcL) allows unchecked proliferation. The inflammatory response further damages the alveolar-capillary membrane, increasing permeability and causing progressive hypoxemia with a widened alveolar-arterial oxygen gradient.