The lungs are an immunologically unique environment; they are exposed to innumerable pathogens and particulate matter daily. Appropriate clearance of pathogens and response to pollutants is required to prevent overwhelming infection, while preventing tissue damage and maintaining efficient gas exchange. Broadly, the innate immune system is the collection of immediate, intrinsic immune responses to pathogen or tissue injury. In this review, we will examine the innate immune responses of the lung, with a particular focus on their role in pneumonia. We will discuss the anatomic barriers and antimicrobial proteins of the lung, pathogen and injury recognition, and the role of leukocytes (macrophages, neutrophils, and innate lymphocytes) and lung stromal cells in innate immunity. Throughout the review, we will focus on new findings in innate immunity as well as features that are unique to the lung.
Keywords: Bacterial pneumonia; Extracellular matrix; Innate immunity; Innate lymphocytes; Lung endothelium; Lung epithelium; Macrophage; Neutrophil; Viral pneumonia.
© 2023. The Author(s).
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Conflict of interest statement
The authors declare that they have no competing interests.
Figures
Overview of features of the innate immune response in the lungs. Multiple immune and non-immune cell types participate in the pulmonary innate immune response. Airway epithelial cells produce mucus and antimicrobial peptides, which traps and neutralizes debris and pathogens, and removes them from the lung via the mucociliary elevator. Resident immune cells, including macrophages and lymphocytes, and alveolar epithelial cells surveil the airway and alveolar spaces for potential pathogens, secreting chemokines, and cytokines to recruit additional effector cells such as neutrophils and monocytes. Migration of these cells to the alveolar space through the extracellular matrix is facilitated by vascular endothelium, fibroblasts, and alveolar epithelial cells
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