MHC class II formation and loading
When a cell engulfs antigenic proteins from outside the cell, the particles are taken into the cytoplasm of the cell in a fluid filled sac, or endosomal vesicle (see uptake page). Here the antigen is processed into small peptide pieces.
While the protein is being dismantled, MHC class II molecules are being made in the ER of the cell. Let's consider the MHC molecules residing in the ER, waiting to be loaded with peptides. How does the cell assure that MHC class II gets loaded only with peptides coming from the external environment? See if you can figure this out by watching this video.
The alpha-chain and beta-chain of the MHC class II molecule becomes associated with another molecule, the invariant chain (Ii). The invariant chain sits in the peptide binding groove and promotes correct folding of the MHC class II and guides the MHC class II molecule into the correct endosomal pathway. In addition, the invariant chain stops peptides from intracellular proteins from binding to MHC class II. The newly made MHC molecule is transported from the ER, via the golgi apparatus, to merge with the endosome where it meets peptide fragments of the antigens. The invariant chain is then cleaved leaving only a small peptide called CLIP associated with the MHC. Another molecule is required to release CLIP from the MHC to allow binding of the correct antigenic peptide. Did you spot which molecule this was? The complex is then taken to the cell surface where it can be presented to T cells.