section_path: Function/Protection
section_name: Protection
paragraph_text: Protection
  
  The lungs possess several characteristics which protect against infection. The respiratory tract is lined by respiratory epithelium or respiratory mucosa, with hair-like projections called cilia that beat rhythmically and carry mucus. This mucociliary clearance is an important defence system against air-borne infection. The dust particles and bacteria in the inhaled air are caught in the mucosal surface of the airways, and are moved up towards the pharynx by the rhythmic upward beating action of the cilia. The lining of the lung also secretes immunoglobulin A which protects against respiratory infections; goblet cells secrete mucus which also contains several antimicrobial compounds such as defensins, antiproteases, and antioxidants. A rare type of specialised cell called a pulmonary ionocyte that is suggested may regulate mucus viscosity has been described. In addition, the lining of the lung also contains macrophages, immune cells which engulf and destroy debris and microbes that enter the lung in a process known as phagocytosis; and dendritic cells which present antigens to activate components of the adaptive immune system such as T cells and B cells.
  
  The size of the respiratory tract and the flow of air also protect the lungs from larger particles. Smaller particles deposit in the mouth and behind the mouth in the oropharynx, and larger particles are trapped in nasal hair after inhalation.