Introduction
The respiratory system has an extensive surface area and is continuously exposed to the external environment through inhaled air. As it remains constantly exposed to the external environment, it is more prone to respiratory disorders. The defense mechanisms of the lung function to prevent dreadful lung diseases. The appearance of a lung disease largely depends on the success or failure of the respiratory defense mechanisms. The immune responses of the body amplify and direct defenses against infections and also offer protection against various harmful agents.
What Is the Role of Defense Mechanisms of the Respiratory System?
The lung is protected by interlocking systems of defense, such as specific and non-specific defense mechanisms. Mechanical barriers isolate inhaled particles or are physically removed from the lung by transport to the lymph nodes through interstitial and lymphatic channels or by transport to the bronchial mucociliary escalator. The inhaled substances are locally detoxified within the lung through interactions with antibodies or by dissolution within the phagocytic cells. The alveolar macrophage in the respiratory membrane operates in nonspecific modes of defense and is also augmented by specific immunologic mechanisms. The effectiveness of pulmonary defense is determined by the variations in macrophage function and physiology.
The alveolar macrophage functions as the resident defender of the alveolus (air sacs of the lungs). The alveolar macrophage is an essential component of the pulmonary immune response and plays a vital role in the pathogenesis of various inflammatory, fibrotic, and destructive lung diseases. Immunoglobulin A and G (IgG and IgA), macrophage-stimulating T-lymphocyte, and microbial-neutralizing antibodies are the vital immunospecific forms of pulmonary defense. The pulmonary defense mechanism may be impaired by various factors, including cigarette smoke, infectious agents, industrial pollutants, air pollutants, a spectrum of coexisting diseases, and an increased susceptibility to acute and chronic respiratory disorders.
What Are the Nonspecific Defense Mechanisms of the Respiratory System?
Nonspecific defense mechanisms of the respiratory system include local detoxification, clearance, and reflex responses.
Clearance:
Clearance refers to the physical removal of the particles deposited on the respiratory tract's surfaces. The mechanism involved in the removal and the time required for clearance depends on the respiratory tract region involved. The mucociliary system is responsible for the clearance of the conducting airways. The nasal passages and the airways of the tracheobronchial tree, except the anterior nares and the posterior nasopharynx, are lined by ciliated epithelium, overlaid by a viscous fluid layer called mucus. The mucus clears the deposited materials from the conducting airways through the movement of the cilia. The coordinated movement of cilia and the movement of the mucus clears the conducting airways.
The alveolar macrophage functions as the first-line defense against bacteria and insoluble particles. It isolates, transports, and detoxifies the deposited particles. The alveolar macrophages are phagocytes that contain an array of proteolytic enzymes. These enzymes digest a wide variety of organic materials and kill bacteria via peroxide-producing oxidative mechanisms. Free particles in the interstitium are engulfed by the macrophages in that site.
Local Detoxification:
The respiratory tract fluids contain various mucopolysaccharides and proteins and are involved in detoxification. Lysozyme, transferrin, and lactoferrin are involved in detoxification activity. Alveolar fluid also includes multiple components that are involved in antimicrobial defense.
Reflex Responses:
Inhaled particles may cause reflex responses due to chemical or mechanical stimulation of receptors in the epithelium of pulmonary parenchyma and bronchi. Some responses may prevent the further entry of particles. These responses include apnea (temporary suppression of breathing), laryngeal constriction, bronchoconstriction, hyperpnea (rapid breathing), or dyspnea (shortness of breath). Sneezing and coughing also clear irritants.
What Are the Specific Defense Mechanisms of the Respiratory System?
Specific defense mechanisms may be antibody-mediated or cell-mediated. Inhaled antigens provoke immunogenic responses and are expressed in the region where the antigens contact the tissues of the respiratory tract. The degree of immunogenic stimulation depends on the properties of the antigen evoking the response. These responses protect the respiratory system against pathogens.
Antigens that enter the airway fluid barrier may reach the lymphatic vessels and contact lymph nodes. Antibody-mediated immune responses are produced by antibody-producing cells called plasma cells. Plasma cells develop from bone marrow-derived lymphocytes, which are present in lymphoid tissues and the lining of the bronchial tree. Plasma cells are produced by antigenic stimulation of bone marrow-derived lymphocytes. These cells secrete antibodies into the fluids of the respiratory tract.
Various immunoglobulins are present in the respiratory tract. Immunoglobulin A (IgA) is the primary immunoglobulin in the upper respiratory tract, and it is also present in the lower respiratory tract to some extent. The primary role of immunoglobulin A is to prevent the invasion of antigens into the cells by preventing the attachment of bacteria to surfaces of the airway and also neutralizing bacterial toxins. Other immunoglobulins, such as IgE, are found in the conducting airways and participate in lung defense. Thymus-derived lymphocytes (T-cells) participate in cellular immunity. These T-cells arise from stem cells in the bone marrow and continuously circulate through the lymphatic system. T-cells proliferate and mediate immune responses during interaction with appropriate antigens. These sensitized T-cells secrete lymphokines, which activate macrophages. Activated macrophages phagocytose (surrounds and engulfs the pathogen), the sensitizing antigen, and the intracellular pathogens. Some T-cells may or may not produce lymphokines. In contrast, other T-cells regulate antibody production by B-cells and provide residual immunity against antigens involved in chronic intracellular infections of the lung.
In the lower airways, lymph nodes are activated, and antibodies are produced if the antigens enter the lower respiratory tract. The antibody-forming cells in lymph nodes enter the lung parenchyma through systemic and lymphatic circulation and produce antibodies at a faster rate in response to the antigens. Studies have shown that more than 30,000 antibodies are produced when B-cells are activated in response to infections.
Conclusion
The respiratory tract is exposed to various toxic substances in ambient air. This includes pathogenic organisms, radioactive or toxic gases, and particles. The surface area of the alveolar region and the relationship between pulmonary circulation and the external environment make the lung more susceptible to the entry of pathogens. An array of specific and nonspecific defense mechanisms in the respiratory system detoxify and remove these pathogens through cellular and acellular processes. Hence, the success or failure of respiratory defense mechanisms largely determines the appearance of lung diseases.
