Leukemic phase of malignant histiocytosis (arguments in favour of the histiomonocytic origin of the abnormal cells).

A 15-year-old female was treated for malignant histiocytosis. The occurrence of a leukemic phase (178,000 blasts/cu mm) during the follow up provided the opportunity of studying a large number of malignant cells by cytochemical electron microscopic, and cell membrane markers techniques. Acid phosphatase reaction was strongly positive and totally inhibited by tartaric acid. Nonspecific esterase reaction was moderately positive with inhibition by sodium fluorid. Electron microscopy revealed the presence of surface membrane pseudopods and the phagocytic activity of the cells. The leukemic cells had a receptor for the Fc fragment of IgG. These findings support the histiomonocytic origin of the abnormal cells in malignant histiocytosis.