Inhibition of NK-mediated killing by homophilic CEACAM1 interactions. Killing of .221 and .221/CEACAM1 cells, incubated with or without polyclonal anti-CEACAM antibodies, by a representative CEACAM1⁺ NK clone (panel A) or by a CEACAM1^– NK clone (panel B). As control, anti–glutathion S-transferase (GST)–ABL polyclonal antibodies were used. The effector-to-target (E/T) ratio was 2:1. All antibodies used were in the form of F(ab′)₂. Figures show the average of 3 independent experiments. The data represent means of the percentage of killing ± SDs.