macrophage
Meaning of macrophage: Half scavenger and half messenger: the cell that eats an invader also decides how loudly the rest of the immune system answers.
Definition of macrophage
A macrophage is a large white blood cell that lives in tissue and swallows microbes, dead cells and debris, then breaks them down inside itself. It also starts and steers the wider immune reaction, displaying fragments of what it has eaten to other immune cells and releasing chemical signals that summon them. Macrophages are present in nearly every organ.
How does a macrophage do its job?
As a phagocyte, it recognizes a target either by surface patterns common to microbes or by the coating of antibody and complement proteins the body applies to mark something for removal. It then wraps the target in a fold of its own membrane, draws it inside, and fuses that pocket with sacs of digestive enzymes. What survives digestion is not discarded. Fragments are placed on the cell surface where T cells can inspect them, which is how a general response becomes a specific one. At the same time the macrophage releases signals that raise body temperature, pull in other white cells, and later switch the reaction off and start repair, so it shapes an immune response from beginning to end.
Where do macrophages come from?
| Population | Where it sits |
|---|---|
| Microglia | Brain and spinal cord |
| Kupffer cells | Liver |
| Alveolar macrophages | Air sacs of the lung |
| Monocyte derived macrophages | Recruited from blood into any tissue during infection or injury |
Older teaching held that every macrophage was a monocyte that had left the blood and matured. That is only part of the picture. Many resident populations are laid down before birth from early precursors and maintain their own numbers locally for life, while monocytes supply the reinforcements that arrive when something goes wrong. The distinction matters because the two groups behave differently and are replaced on different timescales.
What happens when macrophages drive disease?
Their appetite can work against the body. Macrophages that load themselves with fat inside an artery wall become the foam cells at the center of a plaque. When they meet an organism they can ingest but cannot kill, they wall it off with other cells into a granuloma, the structure that defines tuberculosis and sarcoidosis under the microscope. Within a tumor, macrophages can be steered by surrounding signals into supporting growth and building new blood vessels rather than attacking. And when their activating signals run unchecked, the resulting storm of inflammation damages organs directly.
Used in a sentence
Tissue biopsy showed an infiltrate of macrophages consistent with chronic inflammation.
Good to know
Many tissue macrophages, including those of the brain, liver and lung, are established before birth and renew themselves locally, so the older description of every macrophage as a matured circulating monocyte holds only for the populations recruited during infection and injury.
Word origin
Where “macrophage” comes from
Formed from Greek makros, meaning large, and phagein, meaning to eat, describing the cell's role in engulfing and consuming foreign material.