This ninth edition of Cellular and Molecular Immunology includes substantial revisions, which we made to keep the textbook current with scientiﬁc advances and, at the same time, maintain the clear and readable style that has been typical of previous editions. Whenever we have added new information, we have focused primarily on important concepts and have not increased the length of the book. We have also rewritten many sections for increased clarity, accuracy, and completeness. A general theme in modern immunology is that the ﬁeld is moving beyond establishing fundamental principles of the mechanisms of immune responses to applying these principles to understand human disease and develop new therapies. The revolution in immunological therapies over the last twenty years has been extraordinary. It is especially satisfying for immunologists that some of the most innovative and effective immunotherapies have been developed because the basic science has matured and the complex mechanisms of immune activation and regulation have been elucidated in increasing detail. In this edition of the book, we have paid special attention to the clinical relevance of immunology and have emphasized how newly developed therapies work and what their strengths and pitfalls are.