IgG Subclasses of Intercellular and Basement Membrane Zone Antibodies: The Relationship to the Capability of Complement Fixation

There are four main subclasses of human IgG: IgG1, IgG2, IgG3, and IgG4, among which IgG1-IgG3 activate complement, but IgG4 does not. We studied the IgG subclasses of anti-intercellular (IC) antibodies in pemphigus patients and anti-basement membrane zone (BMZ) antibodies in bullous pemphigoid (BP) patients by immunofluorescent staining using mouse monoclonal antibodies against human IgG1-IgG4. At the same time, the capability of complement fixation of each serum was determined by means of complement immunofluorescence. In both pemphigus and BP autoantibodies, various distributions of IgG subclass were shown, but specific patterns were not observed. In BP, all of the complement fixing antibodies had at least one of IgG1-IgG3 subclasses, while noncomplement fixing antibodies only possessed IgG4. This result agreed well with the biologic characteristics of IgG subclasses in respect of complement fixing capability. On the contrary, in pemphigus, the circulating antibodies showed a distribution of IgG subclass which did not correlate with the biologic characteristics in terms of complement activation. This discrepancy may further dispute the roles of the complement system on the bulla formation in pemphigus.