Introduction

The MHC class II (also known as immune response-associated antigens, or Ia) genes are a multigene family encoded on chromosome 6 in humans (Fig. 5.1) and chromosome 17 in mice (see Chapter 2). The functional role of the MHC class II antigens in the immune response is described extensively in Chapters 1 and 3. This includes their function in antigen presentation, thymic education and T cell recognition/activation, their contribution to transplant acceptance or rejection, their genetic association with diseases, their aberrant up-regulation in a number of autoimmune and inflammatory diseases as well as their absence in specific immunodeficiencies. The involvement of MHC class II gene products in a large number of clinical conditions confers practical importance to the study of their regulation. Furthermore, the tissue-, cellspecific, differentiation-dependent and cytokine-regulated expression of MHC class II genes represent features that have made these genes a model system to study gene regulation. In this chapter, the expression and regulation of MHC class II genes in normal and disease states will be discussed, as well as the molecular basis for the constitutive and inducible modes of class II gene regulation.

Role of the MHC class II antigens in the immune response

The specificity of the immune system is conferred by the binding of foreign antigens through receptors on the surfaces of B and T lymphocytes, the immunoglobulin (Ig) molecule and the T cell receptor (TCR), respectively.