A Plaque Technique for Assay and Characterization of Antibody‐Dependent Cytotoxic Effector (K) Cells

Using monolayers of erythrocytes as target cells, a plaque assay was developed by which individual antibody‐dependent cytolytic lymphoid cells (K cells) could be identified at the cellular level. The extent of plaque formation depended on the concentration of sensitizing antibody, time of incubation, and number of lymphocytes added. Most of the plaque‐forming cells (50% to 70%) were shown to possess receptors for activated complement, a small but variable fraction (3% to 25%) had surface Ig detectable by indirect immunofluorescence, and 5% to 10% bound sheep erythrocytes at 4°C. The plaque‐forming cells appeared as villous, small to medium‐sized lymphocytes when studied by transmission or scanning electron microscopy. Under standardized experimental conditions the minimal number of active, plaque‐forming K cells in purified lymphocyte preparations could be estimated.