ABSTRACT
There exist two forms of MHC molecules, i.e., MHC class I and MHC class II, which not only differ in structure but also with regard to their function, quality of peptides bound and their peptide binding characteristics. MHC class II molecule expression is restricted to B cells, dendritic cells and macrophages (so called antigen presenting cells) and present peptides from endocytosed exogenous proteins to CD4+ T lymphocytes which support proliferation and differentiation of specific B cells. MHC class I molecules which, apart from a few exceptions, are expressed by all cells, present mostly endogenous cytosolic and nuclear peptides to CD8+T cells (cytotoxic T lymphocytes-CTLs).1 2
The generation of peptide loaded MHC class I molecules requires in general the proteolytic generation of peptides with a
preferred length of 7-13 amino acids in the cytosol, the efficient transport of peptides via TAP-proteins (transporter associated with antigen processing) into the endoplasmic reticulum (ER) and the assembly of MHC class I trimers and transport to the surface of the plasma membrane.3,4 If a CTL exists that recognizes a given MHC class I allele loaded with a unique viral (foreign) peptide it will proliferate and be activated to destroy the infected cells (Fig. 21.1).
