Immunology is a branch of biomedical sciences to study the immune system physiology in both diseased and healthy states. Some aspects of autoimmunity enable us to understand that it is not always related to pathology. For example, autoimmune reactions are effective in clearing off the unwanted, excess or aged tissues from the body. Also, autoimmunity occurs after the exposure of the non-self-antigen which is structurally similar to the self, assisted by the stimulatory molecules such as cytokines. Therefore, it can be said that there's a minor difference between immunity and auto-immunity. The question of how physiologic immunity changes to pathologic autoimmunity continue to interest researchers. Answer to such questions can be found by understanding physiology of the immune system. This book covers various topics about immunology, its related aspects and pathologies under two sections namely, Immunology of Viruses & Cancers and Basics of Autoimmunity & Multiple Sclerosis. The contributors of this book have carefully selected topics which would be of reader's interests.