Alveolar macrophages

Morphology and origin

Alveolar macrophages belong to the macrophage system, a group of cells united by a common origin from blood monocytes and the capacity for phagocytosis.

They are classified as free (wandering) macrophages due to their ability to migrate between blood and tissues. Their cytoplasm contains vacuoles and granules (phagosomes).

Functions and regulation

• Phagocytosis: cells engulf foreign dust particles and excess degraded surfactant, and in pathology (pulmonary congestion), erythrocytes.
• Interaction with surfactant: surfactant activates alveolar macrophages, regulates their migration from the interstitium, and participates in the opsonization of bacteria, preparing them for phagocytosis.
• Immune response: activated macrophages secrete IL-1, IL-6, IL-12, TNF-α, and produce reactive oxygen metabolites and nitric oxide (NO) to destroy microorganisms.

Role in the infectious process

Alveolar macrophages are target cells for a number of pathogens.

• Intracellular bacteria and fungi: alveolar macrophages are the primary site of replication for facultative intracellular parasites. In tuberculosis, mycobacterial sulfatides inhibit phagosome-lysosome fusion, leading to incomplete phagocytosis. In coccidioidomycosis, fungi block the fusion of phagocytic vacuoles and lysosomes.
• HIV reservoir: alveolar macrophages bear CD4 receptors. HIV exerts minimal cytopathic effect on macrophages; these cells exhibit high resistance to apoptosis and long lifespans (from 2 months). Macrophages undergo productive infection and serve long-term viral reservoirs. The viral protein Nef induces MHC-II molecule degradation and impairs antigen presentation.
• Viral infections: in aerosol infection with hemorrhagic fever with renal syndrome (HFRS), viral particles invade alveolar macrophages and accumulate at the site of entry.

Clinical and diagnostic significance

Assessment of the status and number of alveolar macrophages is important in laboratory diagnostics.

• Sputum analysis: the detection of these cells confirms the quality of the collected sample. This indicates that the sputum was obtained from the deep respiratory tract rather than being saliva or nasopharyngeal mucus.
• Bronchoalveolar lavage (BAL): the method is based on microscopy of washings from the walls of subsegmental bronchi. In a normal cytogram of non-smokers, alveolar macrophages constitute the major cell population (up to 90%).
• «Heart failure cells» (siderophages): these are alveolar macrophages containing hemosiderin. They form as a result of the phagocytosis of erythrocytes that have entered the alveolar lumen during pulmonary venous congestion, such as in left ventricular failure.
• Damaging effect: when activated, alveolar macrophages and polymorphonuclear leukocytes generate large amounts of reactive oxygen species and proteases, which can lead to tissue damage in the respiratory zone.

Class and exam questions

What else to study on this topic

• Pulmonary surfactant
• Bronchoalveolar lavage fluid
• Phagocytosis and antigen presentation
• Heart failure cells (siderophages)

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.