Background: Bullous pemphigoid (BP) is an autoimmune subepidermal blistering disease characterized by autoantibodies against the hemidesmosomal proteins BP180 (type XVII collagen) and BP230. BP not only involves IgG-mediated neutrophil activation, leading to blistering, but also IgE-dependent activation of mast cells and basophils. While IgG and IgE autoantibodies target the extracellular noncollagenous (NC) 16A domain of BP180, little is known whether other BP180 regions are targeted by these antibody classes. Objectives: To characterize IgE and IgG autoantibody binding to antigenic sites on the intracellular domain (ICD) of BP180 compared with BP180 NC16A. Methods: IgE/IgG autoreactivity against recombinant BP180 ICD and NC16A was determined by immunoblotting of sera from 18 patients with BP and 10 controls. Results: Total serum IgE was elevated in 16 of 18 BP sera. Most BP sera tested positive (15 of 18) to NC16A with both immunoglobulin classes. Additionally, 14 of 18 sera showed IgE reactivity with an epitope mapped to the ICD of BP180 (amino acid residues 103-266). Mapping of ICD antigenic sites revealed similar IgE and IgG reactivities for most regions except for greater IgE reactivity to amino acid residues 234-398 (11 of 18 BP sera) than IgG (five of 18). Control sera failed to display IgE reactivity to these antigens. Conclusions: The results indicate that BP180 NC16A is not the only antigenic determinant of IgE autoantibodies in BP and that additional, novel epitopes exist on different regions of the ICD of BP180. The heterogeneous autoimmune response against BP180 suggests intramolecular epitope spreading during disease progression.