1. The formation of soft patches on the mucous membrane of a hollow organ of the body.
2. A form of chronic inflammation principally involving the urinary bladder and characterized by soft, pale, or yellow plaques on its mucosa.

The plaques are composed of macrophages and lymphocytes and an occasional multinucleated giant cell.

Mineralized, laminated concretions (Michaelis Gutmann bodies) and altered bacteria may be present within the macrophages. A localized defect in macrophage function or an abnormal response to coliform bacteria is believed to be the cause.