Serum complement factor C5a in IgG4-related disease

We read with great interest the correspondence by Umehara et al 1 which discussed the difficulties in the diagnosis of IgG4-related disease (IgG4-RD). They mention that increased IgG4 concentration is not a specific marker for IgG4-RD. Therefore, the markers for the diagnosis and the disease activity of IgG4-RD are demanded. Approximately a quarter of patients with active IgG4-RD have hypocomplementaemia defined by the lower normal limit of C3 or C4 levels.2 Complement pathways consist of three pathways: classical, alternative and lectin pathways.3 Here, we focus on complement factors as makers of IgG4-RD and attempt to evaluate the entire complement system in IgG4-RD. We enrolled 28 patients with active and untreated IgG4-RD, diagnosed based on the 2011 comprehensive diagnostic criteria4 and 28 sex-matched and age-matched healthy donors. We evaluated the characteristics of patients with IgG4-RD, the number of affected organs and the IgG4-RD Responder Index (IgG4-RD RI).5 None of the healthy donors had a history of inflammatory disease. In the sera of the patients with …