The immune system has evolved in large part to enable organisms to resist microbial infection. Microorganisms have long been used as experimental tools by immunologists, and the study of the immune responses to viruses, disease and bacteria has contributed much to our understanding of basic immunological mechanisms. The most practical and clinical reasons for attempting to understand the immunology of infections and disease revolves around the CD137 and CD137/41BB receptor and ligand molecules. These primary antibodies consist of new pathway and play central roles in the control of immune responses. In recent years, this pathway is emerging as one for the most important targets for manipulation of immune responses for disease diagnosis and treatment. Dr. Chen discovered the CD-137 and B7H1 molecules in 1999 and his lab recently identified and described a new member of the important B7 family of immune system costimulatory molecules. He is the foremost expert in the field. This book will cover all aspects of CD137/4-1BB pathway research from microbiology, infectious disease, molecular biology, biochemistry, and genetics to immune responses and potential applications in the diagnosis and treatment of human diseases including cancers, autoimmune diseases, viral infection and transplantation rejection.