Editing Respiratory system (section)

====Local defenses====
Irritation of nerve endings within the [[nasal cavity|nasal passages]] or [[airway]]s, can induce a [[cough reflex]] and [[sneezing]]. These responses cause air to be expelled forcefully from the [[Vertebrate trachea|trachea]] or [[nose]], respectively. In this manner, irritants caught in the [[mucus]] which lines the respiratory tract are expelled or moved to the [[mouth]] where they can be [[swallowed]].<ref name=tortora1 /> During coughing, contraction of the smooth muscle in the airway walls narrows the trachea by pulling the ends of the cartilage plates together and by pushing soft tissue into the lumen. This increases the expired airflow rate to dislodge and remove any irritant particle or mucus.

[[Respiratory epithelium]] can secrete a variety of molecules that aid in the defense of the lungs. These include secretory [[immunoglobulin]]s (IgA), [[collectin]]s, [[defensin]]s and other peptides and [[proteases]], [[reactive oxygen species]], and [[reactive nitrogen species]]. These secretions can act directly as antimicrobials to help keep the airway free of infection. A variety of [[chemokine]]s and [[cytokine]]s are also secreted that recruit the traditional immune cells and others to the site of infections.

[[Pulmonary surfactant|Surfactant]] immune function is primarily attributed to two proteins: SP-A and SP-D. These proteins can bind to sugars on the surface of pathogens and thereby [[opsonize]] them for uptake by phagocytes. It also regulates inflammatory responses and interacts with the adaptive immune response. Surfactant degradation or inactivation may contribute to enhanced susceptibility to lung inflammation and infection.<ref>{{cite journal |doi=10.1159/000078172 |pmid=15211087 |title=Host Defense Functions of Pulmonary Surfactant |journal=Biology of the Neonate |volume=85 |issue=4 |pages=326–32 |year=2004 |last1=Wright |first1=Jo Rae |s2cid=25469141 }}</ref>

Most of the respiratory system is lined with mucous membranes that contain [[mucosa-associated lymphoid tissue]], which produces [[white blood cell]]s such as [[lymphocyte]]s.