How do endogenous “foreign” bodies cause granulomas?

Endogenous substances produce a granulomatous reaction when they come in contact with the dermis or subcutaneous fat. For example, one of the most common foreign body reactions occurs when an epidermoid cyst wall ruptures and its keratin contents come in contact with the dermis. Normally, the keratin within the cyst is protected from the dermis by the cyst’s epithelial lining. However, when a cyst ruptures, the keratin is exposed to the dermis and, being a poorly soluble substance, it produces a granulomatous response.

A second mechanism occurs when endogenous substances that are normally soluble crystallize into large aggregates, which then provoke a granulomatous foreign body reaction (e.g., uric acid crystals in gouty tophi and calcium in calcinosis cutis).