Bullous Pemphigoid

Bullous pemphigoid (BP) is an autoimmune blistering disease. IgG autoantibodies target BP180, also called collagen XVII, and BP230—proteins that help anchor the epidermis to the dermis at the dermal–epidermal junction. Antibody binding activates complement and attracts eosinophils. The resulting inflammation separates the epidermis from the dermis, creating a subepidermal blister. The location of the split explains the bedside appearance. Because the entire epidermal roof remains relatively intact, the bullae are usually tense rather than floppy. The surrounding inflammatory response produces marked pruritus and an urticarial or erythematous base. The patient may report intense itching before any blister appears. This pattern differs from pemphigus vulgaris, in which the split is within the epidermis and blisters are typically fragile or flaccid. Prominent mucosal erosions and painful widespread skin loss point away from uncomplicated BP and require reassessment of the differential. Painful dusky or targetoid lesions, rapid epidermal detachment, fever, or systemic illness raise concern for Stevens–Johnson syndrome or toxic epidermal necrolysis rather than ordinary BP.