Granuloma

A granuloma is a compact nodule of immune cells that forms during chronic inflammation when the body confronts something it cannot get rid of, such as a slow-growing microbe or an indigestible foreign particle. Its core is made of activated macrophages called epithelioid cells, so named because they resemble epithelium, and some of these fuse into large multinucleated giant cells. A collar of lymphocytes surrounds the core, and fibrous tissue may later encase the whole cluster.

Granulomas arise through a particular immune pathway. Helper T cells of the Th1 type recognize the persisting antigen and release interferon gamma, which activates macrophages, while macrophage signals such as tumor necrosis factor hold the cluster together. The structure contains the threat but can also damage nearby tissue. In tuberculosis, the center of a granuloma often dies and turns cheese-like, a pattern called caseous necrosis, whereas granulomas without central necrosis are typical of sarcoidosis. Other causes include fungal infections, leprosy, Crohn's disease, some forms of vasculitis and foreign material such as suture or talc.

Finding granulomas in a biopsy narrows the diagnosis, and pathologists then use special stains for mycobacteria and fungi, culture and PCR to pin down the cause. Drugs that block tumor necrosis factor can let granulomas fall apart, which is why people starting them are screened for latent tuberculosis.