In brief, here's how the system works. Large non-toxic particles deposited on the air-conducting trachea and bronchi are propelled upwards by a mucociliary or clearance mechanism. They travel up into the throat and are finally swallowed or expelled by a cough. Smaller particles, those below 10 microns in size that reach the alveoli (gas exchange area), are removed by scavenging cells called macrophanges. The alveoli has the thinnest interface between the blood and the environment in the pulmonary capillaries. In this area scavenging cells transport the particles either toward the clearance channel or through the alveolar wall into the lymphatic vessels that then drain into lymphatic nodes for dispersal or storage.
Very small particles, such as from diesel exhaust, have been shown to carry allergens deep into the lungs to be deposited in the delicate alveolar region. Studies have shown that alveolar macrophanges (scavenging cells) react to mite allergen by producing nitric oxide, which is a biomarker of inflammation in the lungs. This is a surprising event, because mite droppings are too large in size (10 to 50 microns) to enter the alveoli but instead settle on the bronchi. How the mite's allergen entered the alveolar chamber has yet to be fully described, however, some scientists believe that the allergen seeps though the mite's sealed dropping to contaminate objects nearby.
A healthy lung can easily deal with inhaled particles as long as they are not 'overloaded' or toxic in nature. Particles of a toxic or harmful nature present the lungs with a different challenge. A challenge that is too complicated to define in this short article. Suffice to say there are two major immune responses. One reacts to harm from toxic gases or metals and infections, the other is associated with allergy and allergic reaction or invasion by parasites. House dust mites can affect both immune responses but in different ways. Both responses can result in harm to vulnerable lung tissue.
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