Immunoglobulin G4-related disease (IgG4-RD) is a newly recognized disease characterized by elevated serum IgG4 levels, tissue infiltration rich in IgG4+ plasma cells. We report on a case which was first considered as pseudolymphoma from the histopathological analysis, but finally diagnosed as IgG4-related skin lesions. As the morphological features of cutaneous involvement of IgG4-RD are consistent with those of cutaneous pseudolymphoma, we tried immunostaining past potential cases of IgG4-RD. Thirty-two skin specimens (15 men and 17 women; mean age, 53 years) diagnosed as having pseudolymphoma were retrieved from the archives to conduct hematoxylin-eosin, IgG and IgG4 staining. Out of the 32 cases of cutaneous pseudolymphoma, germinal center formation was seen in 22 cases, and moderate-severe fibrosis was seen in seven cases. Eleven cases showed more than 10 IgG4+ plasma cell infiltration/high-power field, and among these 11 cases, seven cases (22%) showed A ratio of IgG4+/IgG+ cells of more than 40%. Thus, out of the 32 cases of cutaneous pseudolymphomas, two cases (6.3%) satisfied IgG4-RD histopathological diagnostic criteria. As clinical presentations and histopathological features of skin involvement of IgG4-RD are analogous to cutaneous B-cell pseudolymphoma, careful identification is required through systemic examination, serum IgG4 measurement and other means.
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