macrophage

Macrophages arise from bone marrow monocytes that circulate in blood and differentiate into tissue-resident macrophages, which have specific names depending on location: Kupffer cells (liver), alveolar macrophages (lung), osteoclasts (bone), microglia (CNS), and Langerhans cells (skin, dendritic-like). They express pattern recognition receptors (e.g., Toll-like receptors) that recognize pathogen-associated molecular patterns (PAMPs), enabling phagocytosis of bacteria, fungi, and cellular debris.

Functionally, macrophages are key antigen-presenting cells, processing and presenting antigens via MHC class II to CD4+ T helper cells, thus linking innate and adaptive immunity. They can be polarized into M1 (classically activated, pro-inflammatory, induced by IFN-gamma and LPS; involved in killing intracellular pathogens via nitric oxide and reactive oxygen species) or M2 (alternatively activated, anti-inflammatory, involved in tissue repair, wound healing, and fibrosis, induced by IL-4/IL-13).

Macrophages secrete numerous cytokines including IL-1, IL-6, TNF-alpha (mediating fever, acute phase response, and systemic inflammation), and IL-12 (promoting Th1 differentiation). They are central to granuloma formation in chronic infections like tuberculosis, where they wall off pathogens they cannot fully destroy, often forming epithelioid cells and multinucleated giant cells under the influence of IFN-gamma.

Clinically, macrophage dysfunction is implicated in chronic granulomatous disease, sarcoidosis, and atherosclerosis (foam cells are lipid-laden macrophages in atherosclerotic plaques). Macrophages also play roles in wound healing, clearing apoptotic cells (efferocytosis), and are exploited by pathogens like Mycobacterium tuberculosis, Leishmania, and HIV (which infects macrophages as a reservoir). Understanding macrophage biology is essential for concepts in immunology, pathology, and pharmacology (e.g., corticosteroids inhibit macrophage function, biologics targeting TNF-alpha).