NK cells can be classified based on their contact/killing history. (A) Venn diagram depicting the distribution of NK cells based on whether they always killed (green), never killed (red), sometimes killed (yellow overlap), or never encountered target cells (white). (B) Contact history of individual NK cells arranged into 5 different classes. The columns represent the number of conjugates formed and each row represents a conjugation/killing sequence. White represents no interaction; red, nonlytic interaction; and green, lytic interaction. Numbers on the left indicate the number of NK cells observed. (C) Frequency of each group as a percentage of the total NK population. (D) Fraction of NK cells that exhibited the indicated killing frequency in the experiments (gray bars) and simulations (black bars). The killing frequency is the fraction of interactions that resulted in target cell death out of all of the interactions in which the NK cell participated. NK cells that never encountered target cells in the experiments or simulation were given a killing frequency of zero and were included in the analysis (thus contributing to the bars at 0.0-0.1).