Processing and presentation of extracellular antigens - The endocytic (MHCII) pathway
Now that we know how potential antigens are taken up by cells, we can focus on what happens next...
We must keep in mind that the ultimate goal when an APC takes in substances is to generate an immune response. In order to do so, the T cells need to be activated and to achieve this they need to specifically recognise the antigen when presented in a special package. This special package is called Major Histocompatibility Complex (MHC) and there are different varieties, but two that are particularly important, class I and class II, associate with antigens from different locations. MHC class-I associates with antigens that originate from within the cell and MHC class-II associates with antigens that originate from the environment surrounding the cell, in other words the antigens that we saw being taken up by the APC in the previous page. Both MHC class-I and II cannot associate with the whole antigen, only a small portion of the original protein, which is called a peptide.
The process of chopping up the original protein into these small peptides is called antigen processing. So how does this happen? You have already seen this happen in the cholesterol video. Take another look and see if you can figure out what it was that digested the LDL proteins into peptides in the lysosome?
After internalisation, extracellular antigens are directed into acidic and enzyme rich endosomal and lysosomal compartments in APCs where peptides are generated. The process of how these extracellular peptides then associate with MHC class II and are presented to T cells will be covered in more detail in later sections, however one thing worth mentioning now is that MHC class II-peptide complexes are only presented to a specific subset of T cell. Find out which one on the next page.