An atypical immunophenotype in generalized eruptive histiocytosis: a case report of coexisting scattered S-100 positivity and seropositivity for Anti-nRNP/Sm antibodies.

Generalized eruptive histiocytosis (GEH) is a rare non-Langerhans cell histiocytosis characterized by multiple symmetrical papular eruptions. We describe an atypical presentation of GEH in a 60-year-old man with scattered S-100 immunopositivity and seropositivity for anti-nRNP/Sm antibodies. Histopathology revealed dense perivascular and interstitial infiltrates of histiocytes, plasma cells, and occasional eosinophils within the upper to mid-dermis, while immunohistochemistry revealed diffuse CD68 positivity, CD1a negativity, and scattered S-100 positivity. The unusual immunophenotype and concurrent abnormal autoimmune serology results suggest a possible association between local histiocytic activation and systemic immune dysregulation. To the best of our knowledge, no previous reports have linked GEH with seropositivity for anti-nRNP/Sm antibodies. The coexistence of this serological finding with an atypical S-100+ immunophenotype in our case provides further insight into the immunopathological features of GEH.