Abul K. Abbas, MBBS, Distinguished Professor and Chair, Department of Pathology, University of California San Francisco, San Francisco, California, Andrew H. H. Lichtman, MD, PhD, Professor of Pathology, Harvard Medical School, Brigham and Women's Hospital, Boston, Massachusetts and Shiv Pillai, MBBS, PhD, Professor of Medicine and Health Sciences and Technology, Harvard Medical School, Ragon Institute of Massachusetts General Hospital, MIT and Harvard, Boston, Massachusetts