1. Leukocyte Biology Section, Medical Research Council and Asthma UK Centre in Allergic Mechanisms of Asthma, National Heart and Lung Institute, Faculty of Medicine, Imperial College London, South Kensington, London SW7 2AZ, U.K.
2. Department of Pathology, School of Dental Medicine, University of Pennsylvania, Philadelphia, PA 19104, U.S.A.
3. Immunology and Infection Section, Division of Cell and Molecular Biology, Faculty of Natural Sciences, Imperial College London, South Kensington, London SW7 2AZ, U.K.