This introductory text has been designed primarily for use in immunology courses for medical, dental, and pharmacy students, other clinical students and undergraduates. It sythesizes the established facts of immunology into a comprehensible and up-to-date account of how the immune system works. Throughout the book the emphasis is upon the human system and how its successes and failures affect the lives of each and every one of us. A concerted effort has been made to break down the barriers which often divide basic and clinical immunology and to use examples drawn from everyday human experience for illustrating immunological principles. Readers need no prior knowledge of immunology or microbiology but some familiarity with basic cell biology and biochemistry would be useful.