Higher frequencies of responsive single Ly49A–expressing NK cells in the presence of a strong versus weak MHC class I ligand. (A) Survival of MHC^−/− splenocytes in D^b and D^d mice relative to MHC⁻ mice. Kinetics over 1, 2, and 4 days. D^d versus D^b: **P < .01 at all time points. depl. indicates depleted. (B) Percentage of CD107a⁺ NK cells expressing the inhibitory receptor Ly49A, but not Ly49C, I, G2, or NKG2A/C/E, from naive D^b and D^d single mice after NKG2D stimulation. One representative experiment of 8. SSC indicates side light scatter. (C) Summary of all experiments performed. **P < .01; ***P < .001.