IgG4 (IgG4-RD) related diseases, with a horizon not limited to Mikulicz's disease.

The IgG4-RD is characterized by pseudotumoral inflammatory lesions caused by lymphoplasmocytic infiltration of IgG4+ cells and elevated serum IgG4. For decades, Mikulicz’s disease and Sjögren’s syndrome were considered identical conditions.1–5 IgG1 immunoglobulin is most prevalent (>50%) and the IgG4 variety constitutes less than 5%. IgG4 has disulfide bonds linking the heavy chains unsteadily, allowing their separation and forming 2 antigen-binding sites, so that the bispecific antibody is asymmetric with an unclear in vivo role. IgG4 interacts with the Fc portion of IgG1–3 and not through the Fab-Fc, as occurs with other immunoglobulins, and has rheumatoid factor activity. Therefore, IgG4 has little or no cross-reactivity between antigens and rarely forms immune complexes having no complement activation capacity.1 IgG4 is characteristically protagonic in: