MHC Molecules

MHC Molecules play a key role in the presentation of antigens.  The foreign protein must be processed into a peptide before it can be presented by MHC. This processing occurs in the proteasome of the cell. There are 2 types of MHC Molecule; Class 1 MHC & Class 2 MHC. 

MHC Class 1

MHC  Class 1 molecules are expressed in all nucleated cells. These are involved in the presentation of endogenous antigens (those found intracellularly).  Presentation of these endogenous antigens is recognised by Cytotoxic T cells that destroy the antigen via a mechanism involving the use of Granzyme and Perfornin.
MHC class 1 molecules have 3 alpha subunits and 1 beta microglobin subunit. Both alpha 1 and 2 form the antigen presenting region of the molecule. MHC class 1 has only one transmembrane domain that anchors it into the cell membrane.

MHC Class 2

MHC Class 2 are expressed in professional antigen presenting cells such as macrophages and B Cells.  They present exogenous antigens (those made outside the cell).  They bind peptides 18-20 amino acids long.

Unlike MHC Class 1 receptors, these molecules have two alpha and two beta subunits, with transmembrane spanning regions from alpha-2 and beta-2. The alpha-1 and beta-1 subunits act as anitgen presenting subunits.