Bullae are large, fluid-filled blisters that occur on the skin and mucous membranes. They are a hallmark of pemphigus vulgaris, a rare autoimmune skin disease that causes the body to produce antibodies that attack the proteins that hold the skin together. This results in the formation of blisters that can break and leave sores that are susceptible to infection. Pemphigus vulgaris is a serious condition that can be life-threatening if not treated properly.