Alveolar macrophage

An alveolar macrophage (or dust cell) is a type of macrophage found in the pulmonary alveolus, near the pneumocytes, but separated from the wall.

Activity of the alveolar macrophage is relatively high, because they are located at one of the major boundaries between the body and the outside world.

Dust cells are another name for monocyte derivatives in the lungs that reside on respiratory surfaces and clean off particles such as dust or microorganisms.

Alveolar macrophages are frequently seen to contain granules of exogenous material such as particulate carbon that they have picked up from respiratory surfaces. Such black granules may be especially common in smoker's lungs or long-term city dwellers.

Inhaled air may contain particles or organisms which would be pathogenic. The respiratory pathway is a prime site for exposure to pathogens and toxic substances. The respiratory tree, comprising the larynx, trachea, and bronchioles, is lined by ciliated epithelia cells that are continually exposed to harmful matter. When these offensive agents infiltrate the superficial barriers, the body's immune system responds in an orchestrated defense involving a litany of specialized cells which target the threat, neutralize it, and clean up the remnants of the battle.

Deep within the lungs exists its constituent alveoli sacs, the sites responsible for the uptake of oxygen and excretion of carbon dioxide. There are three major alveolar cell types in the alveolar wall (pneumocytes):

Type 1 and type 2 pneumocytes. Type 1 pneumocytes (or membranous pneumocytes) form the structure of the alveolus and are responsible for the gas exchange in the alveolus. Type 1 pneumocytes are squamous epithelial cells which are characterized by a superficial layer consisting of large, thin, scale-like cells; they also cover 95% of the alveolar surface, although they are only half as numerous as Type 2 pneumocytes. Type 2 pneumocytes are important in that they can proliferate and differentiate into type 1 pneumocytes, which cannot replicate and are susceptible to a vast numbers of toxic insults. Type 2 pneumocytes are also important because they secrete pulmonary surfactant(PS), which consists 80-90% of phospholipids [(phosophatidylcholine(PC), phosphatidyglycerol(PG), phosphaditylinositol (PI)] and 5-10% of surfactant proteins (SP-A, SP-B, SP-C, AND SP-D). PS is synthesized as lamellar bodies, which are structures consisting of closely packed bilayers that are secreted and then undergo transformation into a morphological form called tubular myelin. PS plays an important role in maintaining normal respiratory mechanics by reducing alveolar surface tension. By lowering alveolar surface tension, PS reduces the energy required to inflate the lungs, and reduces the likelihood of alveolar collapse during expiration. Loosely attached to these alveoli sacs are the alveolar macrophages that protect the lungs from a broad array of microbes and aerosols by devouring and ingesting them through phagocytosis.

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