This work was supported by grants from the Biotechnology and Biological Sciences Research Council, the Medical Research Council, and the Cancer Research Campaign, U.K.  Karen Fraser was the recipient of a William Ross Scholarship from the Cancer Research Campaign, U.K.  We are grateful to the many colleagues who generously provided us with reagents used in this work.  We are especially grateful to Dr. Mark Soloski for providing us with Qa1-transfected human cells and the anti-Qa1 mAbs.  We are grateful to Dr. J. Gray for his skill and advice in the preparation of peptides, to Dr. V. Braud and Dr. C. O’Callaghan for helpful discussions, and to Prof. E. Simpson for constructive comments on the manuscript.