Class I major histocompatibility complex proteins display short peptides, or antigens, derived from normal cell proteins. Peptide-loaded MHC proteins are located on the cell surface where they can be examined by passing T cells of the immune system. The MHC complex has two subunits. The smaller subunit, b2 microglobulin, resembles an immunoglobulin domain. The larger a subunit also has an immunoglobulin-like domain which is linked to a head domain containing the antigen-binding groove. The antigen-binding groove in the MHC head domain is built from two walls composed of long alpha helices that rest on a floor composed of an eight stranded beta sheet. The peptide on display fits snugly between the helices in the groove.The peptide backbone is bound at both ends by highly conserved regions of the MHC protein. Some peptide side chains extend downwards into specific binding pockets in the groove, while other peptide side chains project upwards where they can be recognized by T cells. MHC class I proteins display their bound peptides on the cell surface for immune surveillance. Immune cells, called cytotoxic or killer T cells, for example, express T-cell receptors that bind to the MHC head domain and the bound peptide. If the cell expressing the MHC protein displays a peptide foreign to the immune system, the T cell is activated by this receptor-MHC interaction. The activated T cell then proceeds to destroy the abnormal cell. Cut-away views of this peptide-bound MHC protein complexed with a T-cell receptor reveal the exquisite precision with which the interacting surfaces fit together."