Potential Pitfalls of IgG4 Immunohistochemical Staining on Lesional Tissue in Cutaneous Acantholytic Disorders.

The diagnostic utility of immunohistochemistry on paraffin-embedded sections in bullous disorders is useful when frozen tissue is not available. In pemphigus vulgaris and pemphigus foliaceus, an intercellular lace-like staining pattern of IgG4 on lesional tissue by immunohistochemistry has been described, with a comparable sensitivity and specificity to direct immunofluorescence on perilesional tissue. This study aimed to evaluate the staining pattern of IgG4 in non-immunobullous disorders to highlight the potential pitfalls when using this stain. In this study, we conducted a retrospective review of our institution's database of non-immunobullous disorders where immunohistochemistry of IgG4 was performed to rule out pemphigus. We identified 27 cases where IgG4 immunohistochemistry was performed and observed intercellular IgG4 staining in some cases of Grover disease, bullous impetigo, irritated dermal hypersensitivity reaction, acantholytic actinic keratosis, and graft versus host disease. Our results indicate that the interpretation of IgG4 staining by immunohistochemistry in cutaneous acantholytic disorders should be approached with caution. Confirmation on cryosections with direct immunofluorescence study results is important in these settings.