The swine respiratory system is constantly exposed to environmental pathogens such as viruses, bacteria, and airborne particles. To combat these threats, swine possess highly effective natural defense mechanisms. From physical barriers, such as mucus, to specialized immune responses, these mechanisms work in unison to protect the respiratory tract, maintain animal health, and ensure survival. Understanding these defenses is key to comprehending the physiology and pathophysiology of respiratory diseases.
The combined area of air passages and alveolar surface in the respiratory tract represents the largest epithelial surface of the animal body exposed to the external environment, coupled with the enormous volume of air inspired, which contains both commensal and harmful agents. To withstand these potential aggressions, the system is equipped with robust defenses (ZIMMERMAN et al., 2019). The main defense mechanisms of the swine respiratory system include: filtration and immune defense activity in the nasal turbinate epithelium, the mucociliary apparatus in the respiratory epithelium, and immune defenses mainly related to the lymphoid tissue associated with the bronchi (BALT) and pulmonary macrophages (PALLARÉS et al., 2019).
Most of the respiratory system is lined by respiratory mucosa, with ciliated pseudostratified columnar epithelium containing goblet cells, where the mucociliary apparatus is located (KONING & ERICH, 2016). The mucociliary defense mechanism's primary function is the removal of particles or substances potentially harmful to the respiratory tract, capable of clearing particles larger than 10 μm in diameter before they can reach the bronchial tree. This function is ensured by the ciliary movement and the goblet cells responsible for mucus secretion, which guarantees a continuous flow of infectious and irritating particles aggregated to mucus toward the pharynx, swallowing, and excretion through feces, ensuring ongoing respiratory tract cleaning (TRINDADE et al., 2007; PALLARÉS et al., 2019).
This mechanism is complemented by the reflex action of coughing, which acts to clear the lower airways by propelling mucus, secretions, and other foreign materials accumulated up to the oropharynx. Other defense activities related to infectious diseases in the lower respiratory tract are associated with the presence of bactericidal and bacteriostatic substances in the mucus, which neutralize pathogens, such as lactoferrin, complement, surfactant substances, fibronectin, alpha-1 antitrypsin, and lysozyme.
Additionally, the effect of immune defenses is significant, such as the secretion and action of immunoglobulins A and G, which aid in neutralizing viruses and bacteria (TRINDADE et al., 2007; ZIMMERMAN et al., 2019). Alongside the respiratory mucosa, there are areas of secondary lymphoid tissue associated with the bronchi of the lungs, similar to Peyer's patches in the small intestine, known as "BALT—Bronchus-Associated Lymphoid Tissue." Its proliferation is induced in response to microbial exposure or other types of pulmonary inflammation (TSHERING & PABST, 2000). Some of these clusters may be organized into areas of B lymphocytes, T lymphocytes, and specialized stromal cells, while others are small clusters of B lymphocytes with little organization (PABST, 2007). In this tissue, interaction occurs between lymphocytes and antigens from external pathogens, generating a localized specific response and preventing a systemic immune response (PALLARÉS et al., 2020).
Another crucial defense mechanism involves the action of pulmonary macrophages, which are divided into: alveolar macrophages, in contact with type I pneumocytes (alveolar epithelial cells) and type II pneumocytes (surfactant-producing cells), and interstitial macrophages, located in the interstitial septa between the vascular endothelium and the alveolar epithelium (HUSSEL & BELL, 2014). Alveolar macrophages are the first line of defense against pollutants and pathogens inhaled from the air, with two distinct phenotypes. The M1 phenotype, known as classically activated macrophages, is associated with the inflammatory process, releasing cytokines and recruiting other immune cells to the lungs, while the M2 phenotype contributes to inflammation resolution and tissue repair (VIOLA et al., 2019).
In conclusion, the defense mechanisms of the swine respiratory system play a crucial role in protecting against a wide range of environmental and pathogenic threats. These natural defenses, which include physical barriers like mucus and cilia, as well as immune responses, are essential for maintaining the integrity of the respiratory tract and the overall health of the animals.
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