Immunology is the science dealing with the study of the body defense against foreign invaders as well as against mutagenic and oncogenic threaten. This work is focusing on the study of the immune system in a simplified way for undergraduate medical students as a part of Microbiology subject .The chapters of this book include the introduction to the concept of immune system and the types of immunity. The terminology of medical immunology are well defined. Moreover the structure and functions of the elements of the immune system such as immunoglobulins ,complement system, immune cells ,cytokines ,MHC, and lymphoid organs are mentioned at a basic information .The medical aspects of immune system involving the immunity against microbes, immune tolerance,autoimmunity,transplantation,immunodeficiency,immunization,Immunity against tumors are also basically mentioned. The references for this book are listed at the end of this book