Impressive progress has been made in the general field of immunology which has made possible new understanding and pragmatic approaches to the patient with allergic disease. Indeed, one working in the field of immunology senses a major revolution of immunobiologic thinking, much of which has relevance to the clinical practice of allergy. To the practicing allergist, pediatrician, or internist who must deal with allergic patients, the surging new information may seem confusing and bewildering. As part of our comprehensive series on modern immunobiology which aims to digest this progress, we believe it is appropriate to devote an entire volume to the fundamental principles, new knowledge, and clinical lore on which the modern practice of allergy must be based. In the present volume we strive to bring together relevant contributions from leaders in the field of immunobiology with those whose work stands at the forefront of clinical practice. The advancing understanding has in numerous instances reached the point of clinical application, and we have tried to encompass in this volume the entire scope of modern allergy.