A macrophage is a large phagocytic cell derived from a monocyte that engulfs and digests cellular debris, pathogens and foreign particles as part of the innate immune system.
Explanation
Macrophages are key effectors of the innate immune system and a bridge to adaptive responses. They originate from circulating monocytes that exit the bloodstream and differentiate in tissues. Once resident, they patrol by crawling or extending pseudopodia to survey their microenvironment. Macrophages engulf and digest microbes, apoptotic cells and debris through phagocytosis; ingested material is contained within phagosomes that fuse with lysosomes for degradation. Beyond scavenging, macrophages secrete cytokines and chemokines that recruit and activate other immune cells, and they process and present antigens to T cells, linking innate and adaptive immunity. Different functional states exist: classically activated M1 macrophages are induced by interferon‑γ and microbial products, producing reactive oxygen and nitrogen species and pro‑inflammatory cytokines, while alternatively activated M2 macrophages arise in response to interleukins‑4 and ‑13 and promote tissue repair and anti‑inflammatory responses. Tissue‑specific macrophage populations include Kupffer cells in the liver, microglia in the brain, alveolar macrophages in the lung and osteoclasts in bone. Dysregulated macrophage activity contributes to chronic inflammation and diseases such as atherosclerosis and fibrotic disorders.
Functions and Specializations
- Phagocytosis of bacteria, fungi, parasites and apoptotic cells, followed by enzymatic digestion and antigen presentation.
- Secretion of cytokines (e.g., IL‑1, TNF‑α, IL‑12) and growth factors that orchestrate inflammation and tissue repair.
- Polarization into M1 and M2 phenotypes tailored to defense or healing tasks.
- Tissue‑resident macrophages with specialized roles: Kupffer cells clear blood‑borne pathogens, microglia maintain neural homeostasis, and alveolar macrophages remove inhaled particles.
Macrophages act as versatile sentinels and housekeepers that adapt their behaviour to local signals. Their capacity to ingest, digest and communicate makes them vital for host defence, wound healing and maintenance of tissue homeostasis.
Related Terms: Monocyte, Phagocytosis, Antigen Presentation, Cytokine, Dendritic Cell