Understanding Immunology deals with immunology and its unifying principles, based on the view that the immune system has evolved to combat infectious disease. This book describes the phylogenetic emergence of the immune system; immune reactions in invertebrates and vertebrates; antibody-antigen reactions and the induction of the antibody response; the development of the immune repertoire and self-tolerance; and memory and tolerance in T-cells. This text is organized into 15 chapters and begins with an overview of the immune system, paying particular attention to its basic requirements and properties. This book then discusses antibodies and antigens; the molecular biology of antibody formation; and the role of lymphocytes, lymphoid tissue, and antibody forming cells in the immune response. The following chapters focus on immunocompetent cells and the mechanisms of cell cooperation in the induction of the antibody response, properties of the cells responsible for memory, and the genetic basis of antibody diversity. The reader is also introduced to allelic exclusion and the ontogeny of the immune repertoire; differentiation of T-cells; and cancer and transplantation immunology. The remaining chapters explore aberrations of the immune system and immunity to infectious disease. A comparison of the strategies of vertebrates and invertebrates in adapting to unexpected changes in the environment concludes the book. This book will prove useful as an introduction to immunology to those with some background in biology, particularly, undergraduate or graduate students as well as established researchers in other fields.