Immunology is a distinctive subject that rose in the mid-20th century. The subject developed as scientists started to unravel the mysteries about the defense system against pathogens. Researchers started to understand the mechanisms employed by the innate and the adaptive immune system in defense against pathogens. During the last decade, the subject of immunology has been in sharp focus as the immunotherapies against diseases like cancer and AIDS seems last hope. Employing the body's own defense system against diseases like cancer and AIDS by activating specific cells of the immune system looks promising, and therapies like CAR-T cell therapy have been approved. In the first edition of the book "The Fundamentals of Immunology" we have explained the basics of the defense system of our body. The structure and biology of immunoglobulins, their types and function in antigen binding and antibody dependent cellular cytotoxicity (ADCC) have been described well in chapter six. Focus has been laid on the distinction between an antibody and an immunoglobulin. The structure and function and major histocompatibility complex (MHC) has been described. The education of cells about self and non-self during their maturation and the processing and presentation of antigens by MHC bearing cells and how MHC coordinates both humoral and cell-mediated immune responses has been explained well throughout the book. The book has explained the complement system and its components, mechanisms and functions in a separate chapter. At the end of the book, we have given an insight about the vaccines, their history, development and how they are useful and helpful in the defense against diseases. The book also discusses the immune disfunction and diseases associated with the dysregulation of immune responses.