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, and degraded self-proteins. MHC II molecules present "extracytosolic" antigens, which 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, the endoplasmic reticulin (ER) ribosomes are synthesizing MHC I molecules. 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 significant steps in MHC I processing.
MHC II—Pathogens phagocytosed are brought into a membrane-bound organelle called a phagosome, which then fuses with a lysosome containing enzymes and chemicals that break them 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. The peptide fragment is then transported to the membrane surface.
