Alveolar macrophages

Alveolar macrophages are the cells within the lungs which phagocytose microbes and particulate matter e.g. inhaled dust. They are mainly sited within the alveoli where they arrive via the capillaries after production within bone marrow from monocytes.

Alveolar macrophages are unusual in that they may be:

• fixed: trapped within the connective tissue of the alveolar walls
• free: mobile and scavenge for particles which are trapped within the surfactant layer

The alveolar macrophage may leave the lungs by:

• ascending in the layer of mucus on the 'mucociliary escalator' to the larynx
• passing into alveolar lymphatics

Alveolar macrophages may phagocytose erythrocytes which are extravasated within the lungs as a result of pulmonary congestion. The macrophage then stains positively for iron in the altered pigment haemosiderin. These 'heart failure' cells may be coughed up by the patient in their sputum.