Alveolar macrophages (dust cells) often lie free in the alveolar lumen and can be seen in virtually every microscopic field of the lung, there are two in this field. They are derived from bone marrow monocytes and form part of the macrophage system. They phagocytose particulate matter which may have been inhaled. The macrophages can migrate between neighbouring alveoli through pore in the interalveolar walls (pores of Kohn).
Their normal activity is depressed by irradiation, immunosuppression, cytotoxic drugs and metabolic acidosis.
They are removed from the alveoli by the normal movement of fluid to the larynx. Clearance from the alveoli may be delayed by:
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