English: Mechanisms underlaying the inflammation in respiratory tract. Airway epithelium has complex structure: consists of at least seven diverse cell types interacting with each other by means of tight junctions. Moreover, epithelial calls can deliver the signals into the underlying tissues taking part in the mechanisms of innate and adaptive immune defence. The key transmitters of the signals are dendritic cells. Once pathogenic bacterium (e.g., "S. pneumoniae", "P. aeruginosa") has activated particular pattern recognition receptors on/in epithelial cells, the proinflammatory signaling pathways are activated. This results mainly in IL-1, IL-6 and IL-8 production. These cytokines induce the chemotaxis to the site of infection in its target cells (e.g., neutrophils, dendritic cells and macrophages). On the other hand, representatives of standard microbiota cause only weak signaling preventing the inflammation. The mechanism of distinguishing between harmless and harmful bacteria on the molecular as well as on physiological levels is not completely understood.