The Extensive, Delicate Gas Exchange Surface of an Animal's Lung Is Protected by a Variety of Specific and Nonspecific Defense Mechanisms
When an animal is grazing in a rural environment, the air contains few potentially harmful particles and few pollutant gases. If the animal is intensively housed or is being transported, however, the air may be rife with organic dust that can contain particles of plant and animal origin, infectious agents such as bacteria and viruses, allergens such as spores and pollen, and other agents such as endotoxin.
In addition, there may be pollutant gases such as ammonia, diesel fumes, oxides of nitrogen, and ozone. The respiratory system has a variety of defense mechanisms to protect it against these potentially injurious substances.Nonspecific defenses (also referred to as innate immunity) immediately protect against many inhaled substances.
Nonspecific defenses include the mucociliary system, cough, and the resident phagocytic cells in the alveoli. In addition, toll-like receptors on the surface of many types of cells recognize molecules that are common to many bacteria and fungi. When activated, these receptors immediately initiate mechanisms that lead to expression of proinflammatory cytokines.
Specific defenses involve the immune system and are directed against specific injurious agents, such as a bacterium. Specific defenses need several days to become activated and also have an immune memory that protects against future attacks by the same organism. Respiratory defense mechanisms, which may provide adequate protection to an animal in its pastoral environment, are frequently overwhelmed by the stresses of intensive housing and transportation. When these stresses are severe, for example, the stress produced by transportation, the animal can acquire an acute infectious disease such as pneumonia or pleuritis. Noninfectious stresses that are less severe but more prolonged can lead to chronic airway diseases, such as heaves in horses.