This textbook of basic and clinical immunology has been written primarily for medical and biology students who are receiving their first introduction to this fascinating field. Although we have presumed some knowledge of basic biology (particularly physiology and biochemistry), our primary intent has not been to cover in depth the latest research findings. Rather, we have sought to lay a firm foundation for subsequent reading in the laboratory and clinical sciences: internal medicine, pediatrics, microbiology, serology, physiology, cell biology, and genetics. Hence the first part of the text presents the various components of basic immunology, while the second shows how these elements interact under both normal physiologic and pathologic condi tions. To facilitate comprehension of the relationship between basic and clinical immunology, we have introduced cross-references throughout the book. A glossary of important terms has also been included. Selected references are provided with each chapter to guide the student to additional information on topics of special interest. Throughout the book we have attempted to convey to new students of immunology some of the excitement which the subject has long held for us. If we have succeeded, the task of writing will have been worthwhile.