The Macrophages

The macrophages

Resident tissue macrophages, e.g. Kupffer cells in the liver, alveolar macrophages in the lung, mesangial cells in the kidney, microglial cells in the brain and resident macrophages in the peritoneum and lymph nodes, derive from circulating monocytes which originate in the bone marrow. Tissue macrophages have a number of important functions (see the information box)
and via a range of surface receptors are able to respond in different ways to a wide range of external stimuli. Like neutrophils, resident macrophages can ingest and kill bacteria, but perhaps their major role in acute inflammation is to initiate and orchestrate the inflammatory response by the secretion of important cytokines (see table 1.4) and chemokines. For example, they can secrete large quantities of the neutrophils chemokine IL-8 and other chemokines that specifically attract monocytes to the inflamed site. These monocytes rapidly mature into inflammatory macrophages, which have huge phagocytic and bacterial killing capacity and which also have important scavenging function for damaged microorganisms and proteins and for aged and damaged host cells in the ‘clearing up’ processes during the resolution of the inflammatory response. Finally, resident and inflammatory macrophages can secrete a range of cytokines that are responsible for tissue repair processes, but clearly in effective control of these processes may underlie the excessive fibroproliferative response that characterizes chronic inflammatory diseases such as pyelonephritis and fibrosing alveolitis (see the information box). The Macrophages