Peter Bretscher is a well-known Immunologist. In 1970 he published the Two Signal Model of Lymphocyte Activation, with Melvin Cohn, which provided an explanation for how self-nonself discrimination is realized. This theory has stood the test of time and is a central component, along with the Clonal Selection Theory, of modern immunological thinking. Peter studied physics as an undergraduate at Cambridge University, and then, hoping to find a field offering opportunities for theoretical insight, undertook graduate studies in protein X-ray crystallography in the now famous Cambridge Laboratory of Molecular Biology. He became fascinated at this time by immunology, and was fortunate enough to be able to discuss his early ideas with Francis Crick. This was the beginning of an almost 50 year engagement, during which Peter and his students have made substantial theoretical and experimental contributions to the field. Peter is the author of numerous scientific papers and recently published Rediscovering the Immune System as an Integrated Organ, which looks in detail at the state of immunology today and its relationship to medicine. This book is "an invaluable resource...." (Foreword Clarion Review) and a "must read for anyone interested in immunology, a classic book already." Alexandre Corthay, Head of the Tumor Immunology Group, University of Oslo.