Bullous pemphigoid is a chronic autoimmune disorder that causes large, fluid-filled blisters to form on the skin, primarily affecting older adults. The blisters are usually located on areas of the skin that flex, such as the lower abdomen, upper thighs, or armpits. Bullous pemphigoid occurs when the immune system mistakenly attacks the thin layer of tissue that connects the outer layer of skin (epidermis) to the underlying layer (dermis), leading to inflammation and blister formation.