Malignant histiocytosis with marked cellular atypia--an autopsy case with an immunohistochemical study on neoplastic histiocytes.

A 71-year-old male presenting high fever, pancytopenia, liver dysfunction and jaundice died without a confirmed diagnosis. Microscopically, histiocytes with marked atypia and erythrophagocytosis had infiltrated the spleen, enlarged lymph nodes, liver, and bone marrow. From the above features this case was diagnosed as malignant histiocytosis. The infiltrating histiocytes were classified into three categories: 1) phagocytic cells, 2) atypical, mostly non-phagocytic cells, and 3) bizarre cells including multinucleated giant cells. An immunohistochemical study of histiocyte markers demonstrated that lysozyme was positive in the atypical cells and some phagocytic cells, whereas alpha-l-antitrypsin tended to be localized in the phagocytic cells. Bizarre cells were negative for both markers. No S-100 protein was demonstrated in the neoplastic cells. These immunohistochemical features suggested monocyte-phagocytic origin of the tumor in this case.