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 because they can 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, as well as erythrocytes during pathology (pulmonary congestion).
- 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, and TNF-α, and produce reactive oxygen species and nitric oxide (NO) to destroy microorganisms.
Role in Infectious Processes
Alveolar macrophages serve as target cells for several pathogens.
- Intracellular bacteria and fungi: Alveolar macrophages are the primary replication site 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 express CD4 receptors. HIV exerts minimal cytopathic effects on macrophages; these cells are highly resistant to apoptosis and are long-lived (lasting 2 months or more). Macrophages undergo productive infection and act as long-term viral reservoirs. The viral Nef protein induces MHC-II molecule degradation and impairs antigen presentation.
- Viral infections: During aerosol infection with hantaviruses causing hemorrhagic fever with renal syndrome (HFRS), viral particles invade alveolar macrophages and accumulate at the portal of entry.
Clinical and Diagnostic Significance
Assessing the state and number of alveolar macrophages is important in laboratory diagnostics.
- Sputum analysis: The detection of these cells confirms the quality of the collected specimen, indicating that the sputum originates from the deep respiratory tract rather than being saliva or nasopharyngeal mucus.
- Bronchoalveolar lavage (BAL): This method is based on the microscopy of fluid washed from subsegmental bronchi. In a normal cytogram of nonsmokers, alveolar macrophages constitute the predominant population (up to 90%).
- "Heart failure cells" (siderophages): These are alveolar macrophages containing hemosiderin. They form as a result of phagocytosis of erythrocytes that have leaked into the alveolar lumen during pulmonary venous congestion, such as in left ventricular failure.
- Tissue damage: When activated, alveolar macrophages and polymorphonuclear leukocytes generate high amounts of reactive oxygen species and proteases, which can lead to tissue damage in the respiratory zone.