Abul K. Abbas, MBBS, Distinguished Professor in Pathology, Chair, Department of Pathology, University of California San Francisco, San Francisco, California, Andrew H. Lichtman, MD, PhD, Professor of Pathology, Harvard Medical School, Brigham and Women's Hospital, Boston, Massachusetts, Shiv Pillai, MBBS, PhD, Professor of Medicine and Health Sciences and Technology, Harvard Medical School, Massachusetts General Hospital, Boston, Massachusetts ; illustrations by David L. Baker, MA, Alexandra Baker, MS, CMI, DNA Illustrations, Inc..