Immunoglobulin G4‐related disease and its skin manifestations

Immunoglobulin G4‐related disease (IgG4‐RD) is a systemic inflammatory condition characterized by tissue infiltration with IgG4+ plasma cells and an elevated serum IgG4 level. It usually occurs in middle‐aged or older patients, with male predominance, and can involve multiple organs synchronously or metachronously. Patients with IgG4‐RD generally respond to steroid therapy. IgG4‐related skin lesions are typically erythematous and itchy plaques, subcutaneous nodules, or papules like prurigo nodularis located on the head and neck areas. Histologically, lymphoplasmacytic infiltrates with abundant IgG4+ plasma cells are seen in the dermis and/or subcutaneous tissue. The skin condition of IgG4‐RD does not necessarily exhibit infiltration of IgG4+ plasma cells, which is a prerequisite for IgG4‐related skin disease, but such skin lesions without infiltration by IgG4+ plasma cells can be included in the skin manifestations of IgG4‐RD.