Pemphigus and bullous pemphigoid are autoimmune bullous diseases of the skin. Pemphigus, an intraepidermal blistering disease, is characterized by autoantibodies reactive with antigens located in the intercellular spaces or on the surfaces of epidermal cells. These antibodies, which have recently been shown to activate complement, appear to be the cause of the basic pathologic process of pemphigus, acantholysis. The complement system and the plasminogen-plasmin system may be important mediators in the detachment of epidermal cells. Bullous pemphigoid, a subepidermal blistering disease, is characterized by autoantibodies reactive with an antigen located in the lamina lucida region of the basement membrane zone. These autoantibodies, which will avidly fix complement, appear to mediate subepidermal separation by attraction of a variety of inflammatory cells. Anaphylatoxins, released by activation of C4 and C3, or specific IgE antibodies, may activate mast cells with release of ECF-A attracting eosinophils. With activation of C5, C5a is released which could attract polymorphonuclear leukocytes. Antigen-specific lymphocytes, which can also contribute histamine releasing substances, may also be involved. The exact mechanism by which the epidermis separates from the dermis in bullous pemphigoid, however, remains unresolved.