This text and reference in immunology is now in its Third Edition - revised, updated, and expanded to reflect the growth of information on the immune response, its regulation, and its role in disease pathogenesis. Seven new chapters in the Third Edition focus on advances in characterizing the molecular events that regulate the immune response, understanding the phenomena of antigen processing and presentation, and investigating immune responses to infectious agents. Virtually all the previous edition's chapters have been completely rewritten, many by new contributors, and all chapters include updated references. The expanded section on regulation of the immune response features three new chapters on immunoglobulin class switching; somatic mutation and affinity maturation; and immunosuppression. Antigen processing and presentation, which were only touched on in the previous edition, are now described in a separate chapter in the section on the major histocompatibility complex. Coverage of the role of specific cytokines in the immune response has also been expanded. The revised and expanded section on immunologic mechanisms in disease includes three new chapters on immune responses to infectious agents - viruses, intracellular bacteria, and extracellular bacteria. Chapters on HIV infection, primary immunodeficiency diseases, allergy and hypersensitivity, and immunoparasitology have been rewritten for the Third Edition by new contributors who are authorities on these diseases.