Free cells in the lungs, primarily alveolar macrophages, are apparently derived from both pulmonary and hematopoietic tissues. These cells ingest particles and microorganisms which have deposited on the alveolar surface. Increased deposition of inert or infectious particles may act to recruit additional macrophages. Most free cells containing deposited particles reach the ciliated region of the lungs and are eliminated into the pharynx and swallowed. The yield of cells recovered by multiple lavage of the lung can be used to estimate the numbers of free cells in lungs. If respiratory tract fluid is collected from the trachea, the numbers of free cells excreted from the lungs via the airways can be measured. Better methods for examining the input of free cells and other output pathways need to be developed.
Brain JD. Free Cells in the Lungs: Some Aspects of their Role, Quantitation, and Regulation. Arch Intern Med. 1970;126(3):477–487. doi:10.1001/archinte.1970.00310090107013
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