Figure 1.

Tissue‐resident macrophages persist within lung tissues where they provide protection from inhaled pathogens and allergens but simultaneously prevent overt responses to harmless particulate matter. Interstitial macrophages (IM), which populate the lung interstitium, primarily derive from recruited blood monocytes, although a portion may be resident. At least two distinct subsets can be described, with respective roles in homeostasis or antigen presentation. IMs produce immunoregulatory cytokines both at steady state, and after exposure to environmental stimuli, which allows them to modulate inflammation. Alveolar macrophages (AM) are a self‐renewing population of resident macrophages within the alveolar space that do not rely on replenishment from the bone marrow. Primarily anti‐inflammatory cells, they mediate inflammation and promote homeostasis through phagocytosis of apoptotic cells before lysis and the production of anti‐inflammatory cytokines such as transforming growth factor‐β. AMs also provide protection from inhaled pathogens through phagocytosis and the recruitment of inflammatory cells which clear infections.