The main function of our immune system is to protect us from infection in its various forms, and one of the greatest discoveries in medicine, immunization, was put into use and saved millions of lives one and half century before there were any real scientific understanding of the mechanism involved. But the immune system is a coin with two sides. For a few people, when the coin spins, it falls into the unfavorable side, and then instead of functioning purely as a protective mechanism, immune reactions harmful to the host are produced. Because immune reaction can involve any system of the body, immunology has spread its roots into most branches of medicine. In addition of dealing with harmful conditions resulting from activity of the immune system, clinicians are now more involved than ever in dealing with patients who have diminished immune function. Clearly therefore, an understanding of immunology leads to a wider appreciation of the role of the immune system in health and disease conditions such as tumors and hopefully in the future to improved management through immune therapy.