THE AMPHIPATHIC HELIX AS A STRUCTURAL FEATURE INVOLVED IN T-CELL RECOGNITION

The mammalian immune response to an invasion by a virus, bacteria, protozoa, or other foreign agent involves two distinct, but interdependent mechanisms referred to as humoral response and cellular response. In the humoral response, B cells release antibodies that attach to distinct binding sites on the proteins of the foreign agent, thus enabling phagocytic cells or complement to attach to and eliminate the foreign agent. A central character of antibody binding is that antibodies bind to foreign proteins while the proteins are in their functional, three-dimensional conformation. In contrast, in the cellular immune response, foreign protein is degraded within the host cells; certain fragments of foreign protein are then bound to major histocompatibility (MHC) proteins, and the MHC-fragment complexes migrate to the surface of the host cell where they stimulate a T cell response. The fragment of foreign protein in association with the MHC molecule on the cell surface is crucial for the T cell activation. The host cell that degrades the foreign protein and presents an MHC-fragment complex on its surface is referred to as an antigen-presenting cell (APC); and the foreign protein fragment, typically 8 to 14 consecutive amino acids in the protein chain, is referred to as an antigenic site. A molecule on the T cell that recognizes the MHC fragment complex on the antigen-presenting cell is called the T cell receptor. Although the three-dimensional structure of the T cell receptor is not known, its amino acid sequence shows many similarities with an antibody molecule.