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Morphological and Functional Characterization of Isolated Effector Cells Responsible for Human Natural Killer Activity to Fetal Fibroblasts and to Cultured Cell Line Targets

White cells of human blood (Oldham et al. 1973, Takasugi et al. 1973) and cells recovered from various lymphohematopoietic organs of experimental animals (Herberman et al. 1975a, Kiessling et al. 1975a) display spontaneous cytotoxic activity against various, mostly tumor, target cells in vitro. This cytotoxic activity, taking place in the absence of known presensitization, has been termed natural killer (NK) cytotoxicity. Because the natural killer phenomenon can most readily be demonstrated by using cultured tumor cells as targets, it has been suggested that the phenomenon may be linked to surveillance against cancer (Kiessling et al. 1975c, 1976a, Jondal et al. 1978). Numerous attempts have been made to characterize the effector cells responsible for the NK activity. Fractionation procedures utilizing different surface antigens, surface markers and functional properties of lymphohematopioetic cells have been employed (Herberman et al. 1975b, Kiessling et al. 1975b, Herberman et al. 1977, Kiuchi & Takasugi 1976, Jondal & Pross 1975, Vose & Moore 1977, West et al. 1977, Eremin et al. 1978, Bakacs et al. 1977,