Page 1 Reprinted with permission from Nature 308: 149–153 (1984). Copyright © 1984 Macmillan Publishers Ltd. 2771The Journal of ImmunologyPage 2 2772 PILLARS OF IMMUNOLOGYPage 3 2773The Journal of ImmunologyPage 4 2774 PILLARS OF IMMUNOLOGYPage 5 2775The Journal of Immunology