1. Division of Parasitology, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, United Kingdom
2. National Institute of Biological Standards and Control, Blanche Lane, South Mimms, Potters Bar, Herts EN6 3QG, United Kingdom
3. Division of Molecular Immunology, National Institute for Medical Research, The Ridegway, London NW7 1AA, United Kingdom