1. Institute of Medical Microbiology and Immunology, University of Copenhagen, Blegdamsvej 3C, DK-2200 Copenhagen N, Denmark, Division of Bioengineering and Physical Science, ORS, National Institutes of Health, 13 South Drive, Bethesda, Maryland 20892−5766, Center for Advanced Research in Biotechnology, W. M. Keck Laboratory for Structural Biology, University of Maryland Biotechnology Institute, 9600 Gudelsky Drive, Rockville, Maryland 20850, and Basel Institute for Immunology, Grenzacherstrasse 487, CH...