On the Mayo Clinic Radio program, Dr. S. Vincent Rajkumar, a Mayo Clinic hematologist and researcher, discusses herd immunity, antibodies and how the body works to fight off disease. Dr. Bret Petersen, a Mayo Clinic gastroenterologist, will discuss treatment for bile duct cancer. And Dr. Randall DeMartino, a Mayo Clinic vascular surgeon, will explain peripheral artery disease, a common circulatory problem.