Innate and Adaptive Immunity of the Respiratory System

The respiratory tract is a complex system that provides gas exchange and oxygenation of the blood, while at the same time forming a physical and immunological barrier between the external environment and tissues. Therefore, it is constantly exposed to inhaled agents including allergens, pollutants, commensal, or pathogenic microorganisms. Rapid elimination of these inhaled agents is essential for the maintenance of airway integrity and function. The respiratory immune system is composed of innate and adaptive immune systems. The innate immune system is the first defense to inhaled agents, and nonspecifically identify the microorganisms by recognition of their common microbial motifs. It is composed of epithelial cells, dendritic cells, macrophages, neutrophils, innate lymphoid cells, and monocytes. The adaptive immune response comes into play later than the innate immune response but could not have a protective effect without the support of the local innate immune system. The adaptive immune system involves T cells, including CD8+ T cells, which are mainly in charge of directly killing infected cells, and CD4+ T cells, which mediate the additional stimulation of adaptive cytolytic immunity or B-cell-mediated responses, and B cells, which are involved in the production of antibodies to neutralize pathogens or stimulate additional functional responses in innate cells. As a result, the respiratory system is a highly complex system for human health, interacting with the external environment and playing a critical role as the first-line of defense against lung injury caused by inhaled pathogens and toxins. Maintaining balance in the respiratory tract during injury or infection is crucial to host survival.