Macrophages (Greek: "big eaters") are cells found in tissues that are responsible for phagocytosis of pathogens, dead cells and cellular debris. Macrophages are part of the innate immune system.

Origin

Macrophages are derived from monocytes. Monocytes grow in the bone marrow and enter the bloodstream. Circulating monocytes respond to chemical mediators of inflammation. Upon activation by these mediators, monocytes squeeze through the endothelium. Once through the endothelium, the monocytes are called macrophages. (Some monocytes differentiate into specialized cells such as dendritic cells or microglia).

Function

Phagocytosis

Their main role is the removal of pathogens and necrotic debris. The latter function is more important in chronic inflammation. (The early stages of inflammation are dominated by neutrophil granulocytes.)

Macrophages also present fragments of pathogens (called antigens) that they have ingested with MHC class II molecules on their cell membranes. Helper T cells recognize this and release a lymphokine notification to B cells. The B cells then create and release antibodies specific to the particular antigen, and hence to the pathogen. Macrophages again come into play because they are especially attracted to cells with antibodies attached.

A number of cell types are closely related to macrophages: -

• Dendritic cells (including Langerhans cells)
• Microglia
• Kupffer cells

Activation

This is the process of altering the morphology and functional activity of macrophages so that they become avidly phagocytic. It is initiated by lymphokines , such as the macrophage activation factor (maf) and the macrophage migration-inhibitory factor (mmif), immune complexes, c3b, and various peptides, polysaccharides, and immunologic adjuvants.

The macrophage colony-stimulating factor is a glycoprotein growth factor that causes the committed cell line to proliferate and mature into macrophages.

Role in disease

Due to their role in phagocytosis, macrophages are involved in many diseases of the immune system. For example, they participate in the formation of granulomas, inflammatory lesions that may be caused by a large number of diseases.

Some disorders, mostly rare, of ineffective phagocytosis and macrophage function have been described.

Macrophages are the predominate cells involved in creating the progressive plaque lesions of atherosclerosis.