Epidermolysis bullosa acquisita and bullous systemic lupus erythematosus are blistering skin diseases characterized by IgG autoantibodies that predominantly target the noncollagenous domain 1 of type VII collagen, a skin basement membrane component. The basic immunologic events leading to the blistering processes in these diseases remains unclear. We defined the subclass and light chain compositions of the IgG autoantibodies in 15 patients, in order to gain insight into the blistering mechanism. Immunofluorescence correlated the patients' in vivo-bound and circulating antibasement membrane autoantibodies. Four eukaryotic recombinant proteins, including one full-length and three truncated noncollagenous domain 1 proteins generated by sequential deletion of C-terminal amino acids, were used to perform enzyme-linked immunosorbent assay to detect the patients' anti-type VII collagen autoantibodies. The majority of patients' autoantibodies contained both complement-activating and non-complement-activating IgG subclasses. The presence or absence of complement-activating IgG autoantibody subclasses did not correlate with the inflammatory or noninflammatory clinical phenotype. The majority of tested sera contained both kappa and lambda light chain autoantibodies. All sera that reacted to the full-length noncollagenous domain 1 also reacted to the smallest truncated protein containing the cartilage matrix protein and the first three fibronectinlike repeats. The patients' anti-type VII collagen autoantibodies, likely to be polyclonal in nature, may contribute to the pathogenesis of the blistering process by both complement-dependent inflammatory injury and complement-independent mechanical disruption of the anchoring function of type VII collagen. The N-terminal region of the noncollagenous domain 1 may contain an important antigenic epitope targeted by the IgG autoantibodies.