Macrophage

A MONOCYTE that leaves the BLOOD circulation and takes up residence in the tissues. Once there, the cell undergoes several changes:

• It greatly enlarges.
• It develops pseudopods (footlike projections) that permit it to move through tissue.
• It increases the amount of lysozyme its granules contain, increasing its ability to consume cellular debris.

Macrophages, the IMMUNE SYSTEM’s tissue-based scavengers, are part of the MONONUCLEAR PHAGOCYTE SYSTEM. They engulf, dismantle, and consume the carcasses of cells that other immune cells destroy and of cells that die naturally (APOPTOSIS). They also absorb and breakdown the particulate debris from toxins, BACTERIA, viruses, and other substances. Macrophages also respond to the inflammatory process, contributing to INFLAMMATION and the formation of granulomas. As well, macrophages are key to ANTIGEN presentation and processing, the mechanism by which T-cell lymphocytes and B-cell lymphocytes recognize nonself antigens. As the macrophage dismantles a substance, it displays the substance’s antigens, along with the relevant MAJOR HISTOCOMPATIBILITY COMPLEX (MHC), on the surface of its cell membrane for lymphocytes to detect. Lymphocytes ignore debris that belongs to the body. Debris that is foreign activates an IMMUNE RESPONSE.

For further discussion of macrophages within the context of the structures and functions of the immune system, please see the overview section “The Immune System and Allergies.”

See also B-CELL LYMPHOCYTE; GRANULOMA; LYMPHOCYTE; NATURAL KILLER (NK) CELL; T-CELL LYMPHOCYTE; VIRUS.

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