The major histocompatibility complex (MHC) genes are involved in the immune system’s response to tumour and infected cells and in generating an immune response. This book brings together basic aspects of the regulation of MHC antigens with important clinical applications (in viral infection, viral oncology, cancer biology and autoimmunity). There is a strong emphasis on situations where MHC expression is modulated (either stimulated or repressed). The book’s major themes are: the mechanisms of MHC expression explored at several levels including the transcription and translation of MHC genes and the insertion of MHC protein molecules into plasma membranes; the effect of cytokines on MHC expression both in the aetiology of certain diseases and in possible immunotherapeutic approaches to disease; and the use of gene therapy to modify MHC expression in cancer cells, and thereby cause tumour rejection.