1. Laboratory of Veterinary Immunology, Faculty of Veterinary Medicine, University of Ghent, Salisburylaan 133, B-9820 Merelbeke, Belgium, Laboratory of Physiology and Immunology of Domestic Animals, Faculty of Agricultural and Applied Biological Science, Katholieke Universiteit Leuven, Kasteelpark Arenberg 30, B-3001 Heverlee, Belgium, and Laboratory of Organic Chemistry, Faculty of Science, University of Ghent, Krijgslaan 281 (S4), B-9000 Gent, Belgium