Almost all of our body's cells (except red blood cells and a few others) can process antigens and present them on MHC I molecules. MHC I molecules present "intracytosolic" antigens, such as viruses or intracellular bacteria, as well as self-proteins that have degraded. MHC II molecules present "extracytosolic" antigens - antigens that must first be endocytosed or phagocytosed, broken down, and then presented, such as extracellular pathogens. Only "professional antigen-presenting cells" and a few others can present MHC II molecules. The professional antigen-presenting cells are:
- Dendritic Cells
- Macrophages
- B Cells
Of the three, dendritic cells are the best because, unlike macrophages and B cells, they do not need to be activated first to do this.
Here are the processing pathways in a nutshell:
MHC I - This pathway processes degenerated proteins and intracellular pathogens such as viruses. The object to be processed is first broken down by a large molecule called a "proteasome" into peptide fragments. The proteasome is often likened to a garbage disposal. Meanwhile, MHC I molecules are being synthesized in the ribosomes in the endoplasmic reticulin (ER). Transporter proteins and chaperone proteins facilitate the joining of the peptide fragment into the cleft of the MHC I molecule, which is then transported to the cell's membrane surface. The figure on the right depicts some of the major steps in MHC I processing.
MHC II - Pathogens that have been phagocytosed are brought into a membrane-bound organelle called a phagosome, which then fuses with a lysosome containing enzymes and chemicals that break it down. Meanwhile, MHC II molecules are synthesized in the ER, and chaperone proteins facilitate the joining of the peptide fragment to the MHC II molecule. It is then transported to the membrane surface.
