What specifically (at the molecular level) is the MHC or major histocompatibily complex?

Answer 1

MHC is basically a protein representation that the cell displays to antigen reading cells, such as T-cells, Natural Killer Cells or B-cells.

MHC is a protein created from the DNA of the cell. In the case of Macrophages and other similar immune cells, the MHC presented is a representation of the DNA of the ingested foreign bacteria or virus. There are two classes of MHC. Class 1 is a single stranded protein of sub groups A B and C. It is presented by all cells of the body, and really tells the inspecting cells if the cell is ok or if it is damaged or invaded by a virus (which will implant 'retroposons' onto the normal cell DNA and will show up to an inspecting cell).

The other class... Class 2 is a double stranded protein that is exhibited by 'Antigen Presenting Cells'. This includes macrophages, dendritic cells and some others (like microglia or Kupffner cells). They generally engulf bacteria and display the MHC representing the bacteria to T-cells, or B-cells which take the appropriate action. I may be wrong but I believe that engulfed fragments of destroyed cells of your own body (if attacked by viruses) can also show up the viral representation to the inspecting cells.

I hope this answers your question. I know I've put a lot more in than you asked for, but I am not sure of your level of study and how much you already know. But the nonetheless I hope you find all of this as interesting as I found it!