Case of immunoglobulin G 4‐related skin disease: Possible immunoglobulin G 4‐related skin disease cases in cutaneous pseudolymphoma only by immunohistochemical analysis

Immunoglobulin G4‐related disease ( I g G 4‐ RD ) is a newly recognized disease characterized by elevated serum I g G 4 levels, tissue infiltration rich in I g G 4 + plasma cells. We report on a case which was first considered as pseudolymphoma from the histopathological analysis, but finally diagnosed as I g G 4‐related skin lesions. As the morphological features of cutaneous involvement of I g G 4‐ RD are consistent with those of cutaneous pseudolymphoma, we tried immunostaining past potential cases of I g G 4‐ 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, I g G and I g G 4 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 I g G 4 + plasma cell infiltration/high‐power field, and among these 11 cases, seven cases (22%) showed A ratio of I g G 4 + / I g G + cells of more than 40%. Thus, out of the 32 cases of cutaneous pseudolymphomas, two cases (6.3%) satisfied I g G 4‐ RD histopathological diagnostic criteria. As clinical presentations and histopathological features of skin involvement of I g G 4‐ RD are analogous to cutaneous B‐cell pseudolymphoma, careful identification is required through systemic examination, serum I g G 4 measurement and other means.