The book is designed as a text to cover essential aspects of immunology in a style that makes the subject most exciting and interesting. Emphasis has been to create an understanding of the principles and applications of immunology. Clinical immunology is the study of diseases caused by disorders of the immune system (failure, aberrant action, and malignant growth of the cellular elements of the system). It also involves diseases of other systems, where immune reactions play a part in the pathology and clinical features. Immunology pervades the activities of most clinicians. This is scarcely surprising since so much human disease results from the failure to combat infectious agents or from allergic reactions to antigens in the form of drugs, chemicals, or simple flora and fauna. The importance of immunological mechanisms in the pathogenesis of disease is reflected in the context of prophylactic immunisation or the therapeutic manipulation of the immune response. These intellectual and practical considerations have provided the stimulus for establishing the new discipline of clinical immunology. Immunology began as a branch of microbiology. It was initially understood to be related to infectious diseases alone. The study of infectious disease and the body's response to them has a major role for the development of immunology and its study. This began with the concept of germ theory. This book is presented in a user-friendly mode, aiming to rally the needs of not only the students and teachers but also professional in the field.