1. Molecular Immunology Group and Medical Research Council (MRC) Human Immunology Unit, MRC Weatherall Institute of Molecular Medicine, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DS, UK.
2. MRC International Nutrition Group, London School of Hygiene and Tropical Medicine, London WC1E 7HT, UK, and MRC Keneba, The Gambia.