Macrophage

Overview

Macrophages are the major degradative leukocytes of the immune system and function to scavenge and physically clear tissues of cellular debris and microbes. In doing so, Macrophages are one of the primary effectors of the Innate Immune Response.

Functions

Tissue macrophages exist at baseline throughout nearly all host tissues and serve to recycle dead or dying cellular debris as part of physiological cellular turnover. These cells, which were historically termed "Histiocytes", can also act as immune sentinels and are one of the first host cell types to recognize microbial infection. In general, macrophages phagocytose invading microbes, travel to local lymph nodes, and present the degraded microbial peptides on MHC II which can initiate the Adaptive Immune Response. Consequently, macrophages can act as Antigen Presenting Cells although their role is far weaker than Dendritic Cells in this capacity.

Inflammatory macrophages are recruited to sites of active inflammation and serve to phagocytose and destroy necrotic cellular debris and microbes. Inflammatory macrophages can also act as Antigen Presenting Cells although again, Dendritic Cells are far more important in this capacity.

Development

Both tissue macrophages and inflammatory macrophages are derived from circulating monocytes which adhere and egress into tissues after which they undergo differentiation into macrophages. The development of monocytes is described in more detail in Hematopoiesis.

Subtypes

Specialized names exist for a variety of tissue macrophages in specific tissues.

Morphology

Macrophages characteristically possess a nucleus without lobulation, justifying their classification as a Mononuclear Cell. These cells are often large and may display an amoeboid morphology in tissues. Within areas of active inflammation these cells are easily distinguished by their possession of phagocytosed debris within their cytoplasm.

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