Introduction

T lymphocytes play a central role in antigen-specific immune responses through their interactions with other cell types. These interactions are guided by specific T cell recognition of antigen associated with cell surface molecules of the MHC. The two main types of MHC molecule (class I and class II) are both involved in antigen presentation, but to different types of T cell (expressing CD8 or CD4 antigens, respectively). In addition, they normally show very different patterns of expression within tissues: most cell types of the body express MHC class I antigens, whereas the expression of MHC class II antigens is restricted primarily to cells of the immune system – macrophages, dendritic cells, B lymphocytes and (in humans) activated T lymphocytes. Teleologically, this accords with the usual functions of the T cells, which interact with antigen associated with the different types of MHC protein. Thus, class I-restricted, CD8 cytotoxic T cells may be required to interact with antigen presented by almost any cell of the body (for example, following viral infection). By contrast, a major role of class II-restricted, CD4+ helper T cells is to interact with, and help, other cells of the immune system.

Tissue distribution of MHC antigens

The distribution of MHC antigens is now known to be complex. The expression of class I antigens varies in intensity with, for example, a gradation of expression from strong to weak or negative through the following cell types: cells of the immune system, epithelial cells of the gastrointestinal and respiratory tracts, endocrine cells, striated muscle cells, hepatocytes and neurons (reviewed by Pujol-Borrell & Todd, 1987).