This book is designed as a concept book of Immunology for medical students as well as for graduate students who require a broad introduction to the fundamentals of immunology. It is also suitable for postgraduate courses and competitive entrance examinations including state and all India PGME, USMLE and PLAB. Recent and everyday advances have profoundly influenced the manner in which immunology can and should be learnt. Since immunology first and foremost seeks to explain basics of life and disease, the book has tried to provide the requisite inputs in this direction. There are limited books on basic immunology. The few textbooks available are written by single author. We felt the need of a book which can make immunology understandable; can clear the basic concepts on the subject in simple language and remove the terror of “Immunology being a difficult subject”, from the minds of our students. So, we worked over it and this manuscript is the outcome of the multi-author write-up. Instead of compartmentalizing this interconnected subject, we decentralized different components and correlated it with every organ / part and to every action / response of human body and thus we tried to discuss immunology in its real sense. The invited contributors have expertise and professional involvement in their respective areas. This book differs from the other usual textbooks of immunology in that it provides an overall perception of all the aspects of immunity. The book covers a wide range of topics relevant to medical students beginning from an overview, Innate Immunity, Complement System and Its Receptors, Antigen, Immunoglobulin – Structure and Function, Iso-, Allo- and Idiotypes, Genetics, Antigen-Antibody Interactions, Biogenesis of Antibodies, Cells Involved, Major Histocompatibility Complex and T-Cell Receptors, Response to Antigen and Cell Interaction in Specific Immune Response. Immune Regulation, Immunization, Genetics and Role in Transplantation, Tolerance and Autoimmunity, Hypersensitivity Reactions, Tumor Immunology, Immunodeficiency and Human Immunodeficiency Virus / Acquired Immunodeficiency Syndrome have been sufficiently documented and covered quite comprehensively. The relevance of immunology to society has never been greater; the impact of immunological concepts on advances in understanding medicine, agricultural, environmental sciences, forensic sciences and in various other fields have profound implications for the future of mankind. On one hand, scientists all over the world are trying to conquer “GOD” by trying to understand life completely and advance their capability to alter it while on the other hand, medical and science students want to feel its real and intellectual excitement. Hence, this knowledge of immunology would be of paramount importance for all including well informed citizens.