While the association of particular major histocompatibility complex (MHC) haplotypes with human diseases has been extensively reviewed, we felt that the area of modulation of MHC antigen expression and its association with human and animal diseases was less well explored. These ideas were initially exchanged between the editors and some of the authors of this book at a most interesting meeting organized by the British Society for Immunology in Warwick in 1991 entitled ‘Viruses, Cytokines and the MHC’. Following this meeting, we realised that there was a need for a broad, interdisciplinary treatment of the area of MHC modulation that would be useful for both basic medical researchers and clinicians.

Expression of MHC class I and class II antigens follows a complex pathway from gene transcription to plasma membrane insertion and many steps can be stimulated or repressed leading to altered levels of cell surface MHC molecules. Therefore, to provide basic background information on the MHC, the genomic organization, antigen structure, biosynthesis and function and control of transcription of class I and II are considered first along with the important effects which cytokines and other extracellular agents can have on MHC antigen expression. Infection and oncogenic transformation of mammalian cells by viruses have provided powerful systems for analysing precise mechanisms of MHC antigen modulation and the relationship of this process to disease.