Introduction

Many animal viruses have the property of being able to modulate the expression of the MHC antigens of infected host cells and sometimes of uninfected cells of the host animal. Since the MHC is central to the immune system, this may be beneficial to the virus in evading an immune response and, therefore, have consequences for disease. The significance of such modulation can be seen from how widespread it is, its effects on pathogenicity and its prevalence in common and important human (and animal) infections. It will be shown here and below (Chapters 7 to 12) that modulation of MHC by viruses (and, briefly, other pathogens) is indeed widespread, does affect pathogenicity and is present in common and important infections. It will, therefore, be possible to conclude that modulation of MHC antigens is a highly important characteristic of viruses in general.

This chapter describes the different viruses for which modulation of MHC has been observed. Different mechanisms involved in modulation, both potential and actual, are summarized with suitable examples where known. This information is related to the pathogenicity of the virus where data are available. In the overall context of infectious disease, a brief note is made of other infectious organisms (i.e. microorganisms other than viruses) that are known to affect expression of functional MHC antigens with immunological consequences. Conclusions are drawn from these data on the importance of the modulation of MHC expression and on suitable directions for further research.