CHRONIC INFLAMMATION

If the condition causing acute inflammation is not resolved, the inflammation may pass to a longer term chronic phase. Also, some pathogies by their nature tend to directly provoke chronic rather than acute inflammation.

The primary cells of chronic inflammation are macrophages and lymphocytes. Macrophages live far longer than neutrophils, which last for a few days at most. As their name suggests, macrophages phagocytize pathogens and other material at the site of the inflammation. Because they are long-lived, indigestible material may remain inside macrophages in vesicles for long periods. As noted previously, macrophages are also important secretory cells releasing inflammatory paracrines, growth factors, and a variety of other proteins.

Macrophages are avid phagocytes, and, even if they can't digest all the material phagocytized, they will continue to engulf more. Here is a light micrograph of macrophages distended with lipid from broken down myelin at the site of necrotic tissue due to a blocked blood vessel in the brain. Observe the large amount of nearly clear cytoplasm.

This light micrograph is of brain tissue in multiple sclerosis. Observe the lymphocytes emerging from the venule.

This light micrograph shows thyroiditis in Hashimotos disease, in which chronic inflammation destroys the thyroid gland. Note the lymphocytes.

In certain cases of chronic inflammation, macrophages will collect in layers surrounding the problematical material. Sometimes the macrophages will fuse, forming giant cells. The structure so formed, with layers of macrophages surrounding a central core, is called a granuloma. Granulomas are a characteristic feature of a tuberculosis, in which macrophages can't destroy the phagocytized bacteria, apparently because the bacteria somehow prevent lysosomes from fusing with the phagocytic vesicles