BP often starts with extremely pruritic skin lesions resembling eczema or urticaria before vesicles and blisters arise. In 10-30% of patients, BP also involves the oral mucosa. Disease severity can be determined by means of the autoimmune bullous skin disorder intensity score (ABSIS) that evaluates the involved area as well as the disease activity. The disease is due to an autoimmune response to structural components of junctional adhesion complexes leading to the damage of the dermal-epidermal junction with subepidermal blister formation. Specifically, autoreactive B and T cell responses against the hemidesmosomal antigens BP180 and BP230 have been identified. Serum levels of autoantibodies to BP180 reflect the disease severity and activity. The T cells are memory CD4+ cells producing both Th1 and Th2 cytokines, mostly IL-4, IL-5 and IL-13. IL-5 as well as eotaxin are abundantly found in blister fluids. The production of IL-5 is indeed associated with blood eosinophilia and significant eosinophil infiltration in the skin of BP patients. Eosinophils are thought to be critically implicated in blister formation by releasing toxic granule proteins (ESP, MBP) and proteolytic enzymes.