Macrophages are long-lived phagocytic immune cells found in tissues throughout the body. They originate from different developmental sources, including circulating monocytes that can enter tissues and embryonically derived populations that maintain themselves locally in some organs. Macrophages ingest particles and cells, produce inflammatory and regulatory mediators, present antigen and participate in tissue repair and homeostasis. Their characteristics vary substantially according to anatomical location and environmental signals, so there is no single uniform macrophage phenotype. Tissue macrophages can acquire specialised properties suited to the functions of the organ in which they reside.
Phagocytosis is a central Macrophage function. Macrophages can recognise pathogens, damaged cells and particulate material through pattern-recognition receptors and receptors for opsonins such as antibody or complement fragments. Following engulfment, the target is enclosed within a phagosome that undergoes maturation and fusion with lysosomal compartments. Acidification, hydrolytic enzymes, reactive oxygen and nitrogen-related mechanisms and other antimicrobial processes contribute to degradation. The resulting cellular debris and molecular fragments can then be processed or recycled. Phagocytosis therefore combines recognition, engulfment, intracellular killing or degradation and disposal.
Macrophages participate in inflammation by producing cytokines, chemokines, lipid mediators and other signalling molecules. Depending on the stimulus and tissue context, these mediators can recruit additional immune cells, alter vascular permeability, activate neighbouring cells or promote resolution. Macrophages are often described using the simplified M1 and M2 terminology, with M1 representing classically activated inflammatory macrophages and M2 representing alternatively activated and repair-associated states. Modern immunology recognises that macrophage activation is much more continuous and context-dependent than this binary classification suggests. Real macrophage populations can express combinations of functions influenced by numerous environmental signals.
Macrophages contribute to tissue repair and homeostasis as well as host defence. They can remove dead cells through efferocytosis, regulate inflammation and interact with fibroblasts, endothelial cells and tissue-resident cells during repair. In some circumstances, macrophage-derived mediators support new blood-vessel formation, extracellular-matrix remodelling and restoration of tissue function. These processes can become pathological if macrophage activation persists or is directed towards excessive fibrosis or chronic inflammation. Their capacity to both initiate inflammation and promote resolution makes macrophages important regulators rather than simply destructive immune cells.
Tissue macrophages have specialised names and functions in different organs. Alveolar macrophages reside in the lungs and help clear inhaled particles and regulate pulmonary immune responses. Kupffer cells occupy the liver and participate in clearance and hepatic immune surveillance. Microglia are the resident macrophage-lineage immune cells of the central nervous system, where they monitor tissue and participate in responses to injury and infection. Osteoclasts are specialised multinucleated cells derived from the monocyte-macrophage lineage and resorb bone. These populations share lineage relationships but have distinctive developmental and functional characteristics shaped by their tissue environments.
Macrophages can interact closely with adaptive immune cells. They can present processed peptides on MHC class II molecules to CD4-positive T cells and respond to cytokines produced by T-cell populations. Th1-associated interferon-gamma can enhance macrophage antimicrobial functions, while other cytokines can promote alternative activation and tissue-repair programmes. Macrophages can also be activated by antibody and complement-mediated recognition of targets. Their position at the interface between innate and adaptive immunity allows them to translate signals from pathogens, damaged tissues and lymphocytes into coordinated local responses.
Macrophages can contribute to chronic disease when their normal functions become dysregulated. Persistent macrophage activation is involved in conditions characterised by chronic inflammation, atherosclerotic plaque formation, fibrosis and some autoimmune processes. In tumours, macrophages can adopt phenotypes that support tumour growth, angiogenesis, tissue remodelling or suppression of anti-tumour immunity, although tumour-associated macrophages are heterogeneous rather than a single cell type. Their plasticity makes them both potential therapeutic targets and potential vehicles for targeted treatment strategies.
Macrophages are often confused with monocytes and neutrophils. Monocytes circulate in blood and can differentiate into macrophage-like cells after entering tissues, although many tissue macrophages have embryonic origins and self-renewing populations. Neutrophils are generally shorter-lived circulating granulocytes specialised for rapid responses to acute infection, whereas macrophages are often longer-lived and more involved in sustained tissue surveillance, clearance and repair. Macrophages therefore combine phagocytosis with antigen presentation, inflammatory signalling, resolution and tissue maintenance, making them central components of both defence and homeostasis.
