THE VALUE OF IMMUNOHISTOCHEMISTRY
IN THE ASSESSMENT OF BONE MARROW DISORDERS

Attilio Orazi, M.D., FRCPath. and Dennis P. O'Malley, M.D.

CD1A and S-100 CD1a is the most specific marker for Langerhans cells (LC) and LC histiocytosis that is currently available. The mouse monoclonal O10 antibody which reacts well with conventionally processed bone marrow biopsy can be used to demonstrate LC in bone marrow biopsies. In addition, since CD1a is also expressed in cases of precursor T- cell acute lymphoblastic leukemia/lymphoblastic lymphoma, the antibody can be used to immunophenotype such cases. S-100 although less specific than CD1a can be useful to confirm a diagnosis of LCH. S-100 also stains cases of malignant histiocytosis (CD68 positive, CD1a negative), histiocytes in Rosai-Dorfman disease, NK lymphocytes, and among many solid neoplasms, in particular, cases of malignant melanoma and schwannoma. References Emile et al: Langerhans' cell histiocytosis. Definitive diagnosis with the use of monoclonal antibody O10 on routinely paraffin-embedded samples. Am J Surg Pathol. 1995;19:636-41. Pileri et al: Acute leukaemia immunophenotyping in bone-marrow routine sections. Br J Haematol. 1999;105:394-401.