The role of the lung in immune defense against bacterial aggression

The lung is constantly exposed to the microbial world. This article presents a narrative review of the lung’s immunological role in defense against bacterial pathogens. Although the lung was long considered sterile, recent studies have demonstrated the presence of microbial DNA throughout the respiratory tract. Colonization of the lung begins in utero, as a first step in immunological instruction. The lung microbiome ranges from saprophytic flora to pathogens mostly tolerated by local immunological control. Some host-related factors can impair the body's resistance, with different bacterial agents being identified in community-acquired pneumonia or in pulmonary infections in intensive care units. The arsenal of pulmonary immunity in bacterial infections is based primarily on the respiratory epithelium and a network of immune cells residing in the pulmonary interstitium. The lung can induce protective immunity against respiratory pathogens without activating systemic immunity, functioning as a potent immune organ. The response of pathogens to colonization or infection has been altered by the extensive use of antibiotics and, to some extent, by vaccinations.