Background Information
Major Histocompatibility Complex Class I (abbreviated for obvious reasons as MHC Class I) is a key component of your immune system. The protein is a glycoprotein (protein with a sugar attached) present on all cells in your body. Its function is to show peptides (short stretches of 9-10 amino acids), both self and foreign, to the immune system. This presentation helps the immune system determine whether the body has been infected by a foreign agent (bacteria or virus). When the body is not infected with a pathogen, peptides derived from normal proteins present in your cells are presented on the Class I MHC molecule (these peptides are known as “self” peptides). These peptide-MHC combinations are ignored by the immune system, as the cells of the immune system (mainly T-cells) have been “trained” to not react to self peptides present on the MHC. However, when the body is infected with a pathogen like a virus, cells are commandeered by the virus to produce large amounts of viral proteins that will be used to make new virus. Some of these proteins are broken down by the cell into peptides, and presented on the Class I MHC molecule (these are known as “foreign” peptides). T-cells, the cells that recognize MHC Class I complexed with peptide, are trained to recognize only those MHC Class I molecules that are presenting foreign peptides--such as these protein pieces from the viral invader. The MHC Class I + foreign peptide complex is recognized differently and is key to developing an effective immune response against the pathogenic invader.
As a safety device, the T cell receptor (TCR) on the cytotoxic T cells will only recognize the MHC Class I molecule after it has a foreign peptide bound to it. Since the MHC Class I complexed with self peptides does not trigger an immune response, this keeps the immune system from constantly attacking normal cells, and yet allows the immune system to react quickly to the presence of foreign invading organisms. Autoimmune diseases (such as rheumatoid arthritis and diabetes) often involve the inappropriate recognition of MHC moleules bound to self-peptides, leading to the immune system attacking normal tissue.
You will also encounter MHC molecules if you choose to have yourself tissue typed as a bone marrow or organ donor. There are many different types of MHC molecules - different alleles for the gene; in your body you only use 6 different types and this helps to establish your unique molecular identity - the combination of your six types will certainly be different from your lab mate, but perhaps the same as a family member - remember the joy of genetics.
From a structural point of view, MHC molecules are also cool because you can clearly see alpha helices, beta-pleated sheets, ionic bonds, and quaternary structure. An apt description of the MHC-peptide complex is that it looks like a hot dog (peptide) in a bun (MHC). One other interesting note: the MHC complexes are closely related to immunoglobulins, both being made up of the same basic protein domain motifs (along with the T-cell receptor and several other important immune molecules). These genes all belong to a “superfamily” of related genes, the Ig superfamily.
Two subunits of MHC I (alpha (contain alpha 1, 2 and 3 domains) and Beta-microglobulin)
Sructure
