White lesions of the oral mucosa are generally caused by a condition that increases the thickness of the epithelium. This increases the distance to the vascular bed and thereby tends to change the usual reddish color of the oral mucosa to white. Common causes include hyperkeratosis (thickening of the keratin layer), acanthosis (thickening of the spinous cell layer), increased edema in the epithelium (leukoedema), and reduced vascularity of the underlying lamina propria. Additionally, fibrin caps or surface ulcerations and collapsed bullae can appear white. [from
HPO]