Feed

Severe pneumonia remains a major cause of mortality in critically ill patients, with severe community-acquired pneumonia representing a clinically important and representative scenario. This review focuses primarily on severe community-acquired pneumonia as the clinical context, in which outcomes are determined by the interplay between pathogen burden and heterogeneity of host responses. Beyond pathogen-directed therapies, severe pneumonia is frequently characterized by dynamic and partially overlapping states of hyperinflammation, immune dysfunction, and subsequent immunosuppression. Different host conditions, including immunocompromised states, advanced age, post-sepsis critical illness, and pediatric populations, further increase the complexity and heterogeneity of immune responses. These immune states can be evaluated through conventional inflammatory biomarkers, cellular immune parameters, cytokine profiling, and emerging high-dimensional profiling technologies. This review summarizes the immunopathological basis of severe pneumonia and presents an integrated translational framework encompassing immune monitoring, immune phenotype characterization, risk stratification, evidence-based intervention selection, and serial reassessment. Importantly, immune phenotypic classification should be distinguished from prognostic prediction: models capable of predicting mortality risk do not necessarily identify biologically meaningful immune phenotypes that are amenable to therapeutic intervention. Current evidence supports the use of immune biomarkers primarily as complementary tools for risk stratification and hypothesis-driven patient selection, whereas most immune-targeted interventions remain investigational. Future clinical implementation requires standardized assays, phenotype definitions that incorporate both pathogen characteristics and disease trajectory, external validation, and prospective clinical trials demonstrating improvements in clinically meaningful patient outcomes.