The MHCs: the immunological proteins you probably haven't heard of.

When one thinks of immunological proteins, you think of the big players; histamines, cytokines, and B/T cell receptors like CD4. However there are a little known family of proteins, that do play a vital role in the immune response, more specifically in antigen presentation, which make a bold claim in their name; the Major Histocompatability Complexes. In antigen presentation, a phagocyte like a macrophage or dendritic cell displays the antigens, which are specific peptide sequences used by the immune system to identify a pathogen, from the microbe it has just hydrolysed on its cell membrane. The protein the phagocytes use to do this is the MHC II. The phagocyte presenting these antigens will then travel to a lymphoid organ, the thymus or yellow bone marrow for example, through lymph and activate naive T cells. The CD4 receptor on the T cells must be able to dock to the MHC class II protein, so the epitope; the antigenic determinant which is recognised by the immune system, can imprint on the T cell receptor, priming it and therefore forming the effector T cells: the cytokine releasing T helpers which serve to rally the immune response, or the cytotoxic T killers which kill virally infected cells like homicidal spear wielding warriors. This in fact leads me on to the other class of Major Histocompatability Complex: MHC I. I made a passing reference to it as a 'surface marker' used in the immune response during a previous post, most people would stop there and move on to more significant proteins like interferon, but I'm not like most people (hence this blog). MHC I can be expressed on the surface membrane of almost every body cell, and it also displays the epitopes of antigens when, but for an altogether more sinister purpose... Ok that was slightly dramatic, but I doubt most people will read this far into the entry, so I can do what I want down here (whilst still remaining factually correct of course). The cells displaying epitopes on MHC I are virally infected, and can dock with the CD8 glycoprotein and the TCR found on the surface of T killer cells, and so they release cytotoxins like perforin, which destroys the cell membrane thus promoting PCD by apoptosis. How neat. So without this often overlooked protein, there would be no antigen presentation to trigger the adaptive Immune system, or pleasingly efficient destruction of virally infected cells. It just goes to show how interdependent every molecule in our bodies are, a principle that one should both admire and be absolutely petrified about...