Immunoglobulin deposition and plasma cell infiltration define distinct immunopathological patterns in skin diseases.

Immunoglobulin deposition in the skin is an important diagnostic and etiological feature in many skin diseases. However, the relationship between deposited immunoglobulins and the local presence of antibody-producing cells, such as B cells and plasma cells, remains poorly understood. In several autoimmune diseases, including systemic lupus erythematosus and bullous pemphigoid, marked immunoglobulin deposition is observed despite limited B-cell infiltration into the skin, suggesting that the deposited antibodies primarily originate as circulating autoantibodies. In contrast, diseases such as discoid lupus erythematosus are characterized by progressive B-cell infiltration, the formation of ectopic lymphoid tissue, and increased local antibody production within the skin. In other diseases, including dermatomyositis, a mismatch is observed between circulating autoantibodies and the subclasses of deposited immunoglobulins, suggesting the possibility of local immunoglobulin production independent of systemic reactions. In several inflammatory skin diseases, such as scleroderma, hidradenitis suppurativa, and syphilis, marked plasma cell infiltration is observed from the early stages of the disease, despite weak or irregular immunoglobulin deposition. These findings suggest that plasma cells in the skin have functions other than antibody production and may contribute to disease pathogenesis through cytokine production, immune regulation, and interaction with local tissues.