Figure 24–35 Recognition by T cells of foreign peptides bound to MHC proteins. cytotoxic T cells recognize foreign peptides in association with class I Mhc proteins, whereas helper T cells and regulatory T cells recognize foreign peptides in association with class II Mhc proteins. In both cases, the T cell recognizes the peptide–Mhc complexes on the surface of an Apc—either a dendritic cell or a target cell. Some regulatory T cells recognize self peptides  in association with class II Mhc proteins (not shown).
Figure 24–36 Class I and class II MHC proteins. (A) The α chain of the class I molecule has three extracellular domains, α1, α2, and α3, each encoded by a separate exon. The α chain is noncovalently associated with a smaller polypeptide chain, β2-microglobulin, which is not encoded within the Mhc region of the genome. The α3 domain and β2-microglobulin are Ig-like. While β2-microglobulin is invariant, the α chain is extremely polymorphic, mainly in the α1 and α2 domains. (B) In class II Mhc proteins, both the α chain and the β chain are encoded within the Mhc and are polymorphic, mainly in the α1 and β1 domains; the α2 and β2 domains are Ig-like. Thus, there are striking similarities between class I and class II Mhc proteins. In both, the two outermost domains (shaded in blue) are polymorphic and interact to form a groove that binds peptide fragments. (c) The three-dimensional structure of the peptide-binding groove of a human class I Mhc protein is viewed from above, with bound peptide shown schematically; a peptide must be bound in the groove for the Mhc protein to assemble and be transported to the cell surface. The sides of the groove are formed by two α helices, and the floor is formed by a β pleated sheet. The S–S disulfide bond is shown in red (Movie 24.8 and Movie 24.9). (c, adapted from p.J. Bjorkman et al., Nature 329:506–512, 1987. With permission from Macmillan publishers Ltd.) them have been either eliminated or inactivated, or suppressed by regulatory T cells in the process of self-tolerance. By contrast, in a cell infected by a pathogen such as a virus, the pathogen proteins will be processed in the same way, and peptides derived from them will be displayed on the infected cell surface bound to class I MHC proteins; there, they are recognized by cytotoxic T cells expressing the appropriate TCRs, thereby targeting the infected cell for destruction (Figure 24–38).
In general, only antigen-presenting cells (APCs) express class II MHC proteins. Dendritic cells are referred to as professional APCs, as they are specialized for this function and only they can activate naïve T cells. Other immune cells that are targets of effector T cell regulation, including B cells and macrophages, are nonprofessional APCs. All APCs load their newly synthesized class II MHC proteins with peptides derived mainly from extracellular proteins that are endocytosed and delivered to endosomes. The newly synthesized class II MHC proteins initially contain an invariant chain, which occupies the peptide-binding groove