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.