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Granuloma
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A granuloma is a group of defense cells, or immune cells, gathered at a single spot and packed tightly together. It forms when something irritates the tissue and does not go away: an infectious pathogen, meaning a microbe capable of causing infection, or a foreign body. Its defining feature is mature macrophages arranged in a compact cluster.
Macrophages are defense cells that try to get rid of whatever is causing the inflammation. When they cannot, a larger response may kick in: more macrophages arrive, the inflammation becomes chronic, meaning it lingers, and they all end up crowded tightly around that stimulus. Sometimes they are also ringed by lymphocytes, another type of immune cell. It is a bit like a crowd of people huddling around a rock that no one can move out of the way.
Under the microscope, a granuloma has its own distinctive features. Activated macrophages can change their appearance and look epithelioid, meaning they resemble the cells that line a surface. And when several macrophages fuse together, they form a multinucleated giant cell: a single very large cell with several nuclei.
In brief
A granuloma is a focal cluster of immune cells that forms in response to a persistent inflammatory stimulus, such as an infectious pathogen or a foreign body, with mature macrophages tightly packed together.
If macrophages cannot clear the stimulus effectively, a larger response may kick in: more macrophages arrive, chronic inflammation sets in, and they crowd tightly around the stimulus, sometimes ringed by lymphocytes.
A tight huddle around something that would not go away.
Activated macrophages can become epithelioid, and several fusing together form multinucleated giant cells.
Sources (1)
- National Center for Biotechnology Information, StatPearls PublishingGranuloma, StatPearlsPublished
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