1. Department of Immunology, Division of Medicine, Imperial College London, Hammersmith Hospital, Du Cane Road, London W12 0NN, UK
2. CoMPLEX (Centre for Mathematics and Physical Sciences in the Life Sciences and Experimental Biology), University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, UK
3. Department of Mathematics, Imperial College London, 180 Queen's Gate, London SW7 2BZ, UK