Using an agarose single-cell cytotoxicity assay simultaneously with the antibody-coated ox erythrocyte assay for detecting Fc receptor (FcR)-positive cells, the number of natural killer (NK) cells in fresh human peripheral blood which bear FcR for IgG or IgM have been determined. Of the target binding blood which bear FcR for IgG or IgM have been determined. Of the target binding cells (TBC) which form conjugates with NK-sensitive target cell lines Molt 4, 1301 and Hd-Mar, 55% had FcR for IgG and 24% had FcR for IgM. In the active NK cell population, which amounts to about 6% in nylon wool column-passed peripheral lymphocytes, cells with FcR for IgG account for 60% and cells with FcR for IgM for 17% of the total killing. Even though there is an increase in the number of FcR-IgM-positive cells from fresh peripheral blood after overnight incubation at 37 degrees C, no increase was found in FcR-IgM-positive NK cells. The reasons why FcR for IgM have not been described earlier on human NK cells are discussed, as well as the functional implications of these and their probable expression on other types of cytotoxic effector cells.
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