**2. Bronchi immunology**

The lower respiratory tract is constantly exposed to a wide variety of airborne foreign bodies because it is in direct communication with the external environment for gas exchange [6]. Both the trachea and bronchi function as filters against this exposure due to some of their structural features. The bronchial epithelium has a similar histological structure to the trachea and can capture foreign bodies through the smear of the mucus film secreted by the goblet cells to the kinocilium at the apical ends of the prismatic cells present in its structure. These bodies are captured and removed from the lungs by the movement of the kinocilium toward the larynx [7]. Mechanical filtering of inhaled air is thus ensured due to this primary defense mechanism.

The lower respiratory tract is constantly exposed to allergens, antigens, bacteria, and viruses during gas exchange. This is a very sensitive area for various types of pathogen invasions, such as influenza virus, measles virus, and *Mycobacterium tuberculosis* [8]. Producing rapid and specific adaptive immune responses against these factors are important for survival [6]. At the initial stage of an adaptive immune response, naive T cells migrate through the endothelial venules of blood vessels to secondary lymphoid tissues, where they are stimulated by antigen-bearing cells. This is critical for the development of appropriate adaptive immunity. This migration therefore leads to the generation of antigen-specific effector and memory T and B cells released from the secondary lymphoid tissue into the bloodstream. In the effector stage of the adaptive immune response, some memory T and B cells migrate from blood vessels to non-lymphoid tissues containing cognate antigens or pathogens [9, 10]. Bronchial lymphoid tissue and lymphatic nodes, two types of secondary lymphoid tissue found in the bronchial mucosa of the lower respiratory tract, are important in this regard [11, 12]. These secondary lymphoid tissues play a key role in the development of bronchopulmonary immune responses. Therefore, the bronchopulmonary adaptive immune system plays an important role in maintaining health as well as in the development of lower respiratory tract diseases [6].

The tracheobronchial tree, which is considered as an immunological organ, [13] is important for the defense mechanism of microorganisms reaching the lungs through inhaled air as well as for hypersensitive reactions that occur through respiration. The lymphoid tissue of the tracheobronchial system contains specialized diffuse, clustered, and solitary lymphatic nodules known as bronchus-associated lymphoid tissue [14, 15]. This secondary lymphoid tissue is a representative of the mucosal immune system in the bronchial wall, which is common in different parts of the body. It forms the immunoglobulins as a result of the immune defense reaction, thus forming a special protective mechanism of the lower respiratory system.
