New Evidence Relating to the Nature and Origin of the Hairy Cell of Leukaemic Reticuloendotheliosis

S ummary . New evidence is presented concerning the nature and origin of the pathognomonic cell of leukaemic reticuloendotheliosis (LRE). A specific receptor for IgM was demonstrated by an EA IgM rosette test in all seven cases of LRE studied. This marker was absent from the pathological cells of a wide variety of haematological proliferative disorders, and from cells of the normal myeloid and monocyte‐histiocyte series. A small percentage of circulating mononuclear cells which did not resemble monocytes morphologically, did possess the receptor for IgM. The LRE cells were found to show variable phagocytic activity for IgG opsonized erythro‐cytes and latex particles. Studies of differences in the kinetics of the rosette test in the seven cases together with the range of phagocytic activity and cytological maturity observed, suggested that the diagnosis of LRE may encompass a broader spectrum of clinical and cytological features than has previously been recognized. Evidence presented suggests that the cells of LRE are not related to either B‐lymphocytes or the monocyte‐histiocyte series. It is proposed that LRE represents a leukaemic proliferation of the newly identified small population of EA IgM receptor bearing normal circulating mononuclear cells.